The braking system is one of the most critical safety features on any vehicle, yet its fundamental purpose is often misunderstood. It is commonly believed that a brake system exists to slow or halt the motion of a vehicle, but this is not entirely accurate. The friction of the tires against the road is what actually slows down and stops a vehicle. The brake system serves a different but equally important function: it slows or stops the rotation of the wheels. This distinction extends the responsibility for braking to the tires as well as the brake system itself, highlighting how multiple vehicle components must work together to achieve safe stopping.
The brake system converts the momentum of the vehicle into heat by slowing and stopping the vehicle's wheels. This conversion occurs through friction at the wheels, with the application of the friction units controlled by a hydraulic system. The fundamental principles of all brake systems share common elements, and the hydraulic systems required to stop a vehicle are both sophisticated and essential to modern automotive safety. Understanding how brake systems function requires examining the physics of friction, the various components that make up the system, the hydraulic principles that enable force multiplication, and the different types of brake assemblies and assist mechanisms currently in use.
The Physics of Friction in Braking:
Types of Friction:
There are two basic types of friction that explain how brake systems work: kinetic friction and static friction. Kinetic friction occurs between moving surfaces while static friction operates between stationary surfaces. The amount of friction or resistance to movement, depends on several factors including the type of materials in contact, the smoothness of their rubbing surfaces and the pressure holding them together which is often determined by gravity or weight. Friction always converts moving energy into heat. The greater the friction between two moving surfaces, the greater the amount of heat produced. When the brakes on a moving automobile are applied, rough textured pads or shoes are pressed against rotating parts of the vehicle which can be either rotors for disc brakes or drums for drum brake systems. The kinetic energy or momentum, of the vehicle is then converted into heat energy through the kinetic friction of rubbing surfaces causing the car or truck to slow down. When the vehicle comes to a complete stop, it is held in place by static friction. The friction between the surfaces of the brakes and between the tires and the road resists any movement, keeping the vehicle stationary until the driver releases the brakes or applies power to overcome this resistance.
Factors Governing Braking Power:
Four basic factors determine the braking power of a system. The first three factors govern the generation of friction: pressure, coefficient of friction and frictional contact surface. The fourth factor is a result of friction and involves heat dissipation. An additional factor that influences how well a vehicle will stop when the brakes are applied is weight transfer. When the brakes are applied while the vehicle is moving forward, the weight of the vehicle shifts forward, causing the front of the vehicle to drop, a phenomenon commonly referred to as nose dive. This weight transfer means that the front brakes will need the most stopping power. If the vehicle is overloaded or if the front suspension is weak, more weight will be thrown forward and the brakes will need to work harder.
Pressure:
The amount of friction generated between moving surfaces in contact with each other depends in part on the pressure exerted on those surfaces. The more pressure applied, the greater the resistance to movement. In a brake system, hydraulic systems provide application pressure. Hydraulic force is used to move brake pads or brake shoes against spinning rotors or drums mounted to the wheels. The amount of pressure is determined by the pressure on the brake pedal and the design of the brake system.
Coefficient of Friction:
The amount of friction generated between two surfaces is expressed as a coefficient of friction (COF). The COF is determined by dividing the force required to pull an object across a surface by the weight of the object. For example, if it requires 100 pounds of pull to slide a 100 pound metal part across a concrete floor, the COF is 100 ÷ 100 = 1. To pull a 100 pound block of ice across the same surface may require only 2 pounds of pull, resulting in a COF of 0.02. As it applies to automotive brakes, the COF expresses the frictional relationship between pads and rotors or shoes and drums. The required COF depends on the vehicle and other factors and is carefully chosen by the manufacturer to ensure safe and reliable braking. When replacing pads or shoes, it is important to use replacement parts with a similar COF. If the COF is too high, the brakes will be too sticky to stop the car smoothly, resulting in premature wheel lockup or grabbing. If the coefficient is too low, the friction material tends to slide over the surface of the drum or rotor rather than slowing it down. Most automotive friction materials are engineered with a COF between 0.25 and 0.55.
Frictional Contact Surface:
The third factor governing braking power is the amount of surface area that is in contact between the friction materials and the rotating components. Simply put, bigger brakes stop a car more quickly than smaller brakes used on the same car. For the most part, the vehicle's weight and potential speed determines the size of the friction surface areas. Heavier vehicles and those capable of higher speeds require larger brake components to provide adequate stopping power. Additionally, the greater the surface areas of the wheel brake units, the faster heat can be dissipated, which is crucial for maintaining consistent braking performance over extended use.
Heat Dissipation and Brake Fade:
Any braking system must be able to effectively handle the heat created by friction within the system. The tremendous heat created by the rubbing brake surfaces must be conducted away from the pad and rotor or shoe and drum and be absorbed by the air. Brakes that do not effectively dissipate heat experience brake fade during hard, continuous braking. Brake fade is a condition where the stopping power of the brakes has been drastically reduced, commonly caused by excessive heat buildup. With brake fade, the brake pedal seems normal but there is reduced stopping ability. Brake fade may become worse as heat builds up, potentially due to outgassing. As the shoes or pads become extremely hot, they can generate a gas that can become an air bearing between the frictional material and the rotor or drum. Rather than clamping on the wheel brake, the friction elements will slip on the gas buildup. Fade can also be caused by overheating the brake fluid because gases form in the fluid. The friction materials must be able to dissipate heat and the system must be designed to allow the material to get rid of its heat. This may be done by allowing ample airflow past the brake units. Another way is to ventilate the rotors, which have internal vanes that move hot air from the disc to the outside. Some rotors are cross drilled or slotted, with both designs allowing the rotor to run cooler and reducing the chances of gas buildup. Heat can also cause the linings of the pads and shoes to become glazed and harden the rotor and drum, reducing the COF and requiring excessive foot pressure to be applied to the brake pedal to produce the desired braking effect.
Brake Lining Friction Materials:
Brake linings are made of relatively soft but tough and heat resistant material with a high coefficient of friction. The lining is typically attached to a metal backing with rivets or high temperature adhesives. For many decades asbestos was the standard brake lining material, offering good friction qualities long wear, and low noise. However new materials are being used because of the health hazards of breathing asbestos dust and asbestos has not been used in brake linings or pads since 2003. Many different materials are used as lining material with each type defined by its composition. Each type has different heat dissipation, fade resistance, rotor wear, noise generation and braking force characteristics.
Nonasbestos Organic (NAO) Linings:
They are installed on many vehicles by the original equipment manufacturer. Organic linings are made of nonmetallic fibers bonded together to form a composite material. Today's organic brake linings contain friction materials and modifiers such as graphite, powdered metals and nut shells, fillers for noise reduction and heat transfer binders that hold materials together and curing agents that accelerate chemical reactions. Organic linings have a high COF are economical, quiet, wear slowly and are only mildly abrasive to drums. However they fade more quickly than other materials and do not operate well at high temperatures. High temperature organic linings are available for high performance use but they do not work as well at low temperatures and wear faster than regular organic linings.
Metallic Linings:
They were used for many years in racing applications. Fully metallic lining is made of powdered metal formed into blocks by heat and pressure. These materials provide excellent resistance to brake fade but require high brake pedal pressure and create the most wear on rotors and drums. They work very poorly until fully warmed. Improved high‑temperature organic linings and semimetallic materials have made metallic linings almost obsolete for late model automotive use. Metallic linings are also extremely noisy.
Semimetallic Linings:
They are made of a mixture of organic or synthetic fibers and certain metals molded together. They are harder and more fade resistant than organic materials but require higher brake pedal effort. Most semimetallic linings contain about 50% iron and steel fibers. Copper also has been used in some semimetallic linings and in smaller amounts, in organic linings but concerns about copper contamination of water systems has led to its reduced use. Semimetallic linings operate best above 200°F and must be warmed up to bring them into full efficiency, making them less efficient at low temperatures. They were sometimes used on older heavy or high performance vehicles with four wheel drum brakes. Currently, they are used only on front disc brakes of passenger cars and light trucks. The lighter braking loads on rear brakes, particularly on FWD cars may never heat them to required efficiency. Semimetallic linings also have a lower static COF than organic linings, making them less efficient with parking brakes.
Synthetic Linings:
They are classified as nonorganic, nonmetallic and nonasbestos. Two types are commonly used for drum brakes: fiberglass and aramid fibers. Fiberglass was introduced to help eliminate asbestos offering good heat resistance, good COF and excellent structural strength. Disadvantages include higher cost and reduced friction at very high temperatures. Overall fiberglass linings perform similarly to organic linings and are used primarily in rear drum brakes. Aramid fibers are a family of synthetic materials five times stronger than steel, pound for pound, but weigh little more than half of an equal volume of fiberglass. They have a COF similar to semimetallic linings when cold and close to organic when hot. Overall, aramid linings perform between organic and semimetallic materials but with much better wear resistance and longevity than organic materials.
Carbon Metallic and Ceramic Linings:
They are found on many FWD vehicles because they have high heat resistance. Most ceramic pads are made of ceramic material mixed with copper fibers. These pads are quiet and produce little dust. Carbon mixed with metals yields a lining with good COF and high heat resistance, able to withstand very high temperatures without brake fade. Aftermarket companies offer linings made of carbon, Kevlar and other materials with ceramic heat shields that reduce heat transfer. Some high performance cars use carbon ceramic pads, comprised of a ceramic composite of carbon fiber reinforced with silicon carbide, offering excellent braking performance, extreme light weight and consistent COF across a wide range of temperatures and weather conditions.
Principles of Hydraulic Brake Systems:
A hydraulic system uses brake fluid to transfer pressure from the brake pedal to the pads or shoes. This transfer is reliable because liquids are not compressible. Pressure applied to a liquid in a closed system is transmitted equally to every other part. For example, applying 5 psi through the master cylinder means 5 psi can be measured anywhere in the lines and at each wheel. Force can be increased at the output by increasing the wheel piston's size, though piston travel decreases. To double output force from 5 psi to 10 psi, use a wheel cylinder piston with 2 square inches of area. To triple 100 psi use a piston with 3 square inches producing 300 pounds output. In actual practice, fluid movement is very slight. In an emergency with pedal to the floor, fluid displaced amounts to only about 20 cubic centimeters: 15 cc to front discs and 5 cc to rear drums. The hydraulic system transmits the driver's foot action to the wheels where friction pads are forced against rotors or drums. Mechanical force is changed to hydraulic pressure and back to mechanical force. The force acting on friction pads equals pedal psi multiplied by piston area. For instance, 25 pounds applied to the pedal times 4 square inches of piston area equals 100 psi in the system.
Dual Braking Systems:
Since 1967, federal law has required all cars to have two separate brake systems. If one circuit fails, the other provides enough braking power to stop safely. The dual system employs a tandem master cylinder essentially two master cylinders with two separate pistons and fluid reservoirs in one bore. Each piston applies hydraulic pressure to two wheels. In early dual systems, circuits were separated front and rear. If one failed, the other was available, but front brakes do approximately 70% of braking work, so a front failure leaves only 20–40% braking power. This was reduced with diagonally split systems, where the hydraulic lines are split left front to right rear and right front to left rear. In case of failure, the remaining system provides braking on one front and one rear wheel maintaining 50% of total braking force.
Hydraulic Brake System Components:
Brake Fluid:
Brake fluid is the lifeblood of the system. It must flow freely at 500°F and at ‑104°F. It lubricates parts, fights corrosion and rust, and resists evaporation. All brake fluids are hygroscopic, absorbing water. Moisture can enter when exposed to air or through condensation from temperature changes. Today's vehicles have little airflow under the hood, increasing fluid heat. Moisture lowers the boiling point, causing vapor buildup that leads to brake failure or spongy pedal because vapor is compressible. Moisture also increases viscosity at low temperatures, impairing cold weather braking. Corrosion from moisture decreases efficiency. Tests show that within 1 year of service, water content is about 2%, it takes about 2 years for moisture to lower boiling point to dangerous levels. Brake fluid must be compatible with system materials, providing controlled swell to seals. Every can carries SAE and DOT letters indicating blend and performance. Always use manufacturer‑recommended fluid. Most vehicles have fluid level sensors that trigger a warning when fluid drops below normal, appearing as Brake fluid low on the instrument panel about 15 seconds after the circuit closes.
Brake Pedal:
The brake pedal is where the hydraulic system starts. When depressed, force is applied to the master cylinder. On a basic system without power assist, force is transmitted mechanically. As the pedal pivots, force is multiplied mechanically, so the pushrod force on the master cylinder piston is much greater than the pedal force.
Master Cylinders:
The master cylinder transmits brake pedal pressure to all four wheel brakes. It changes mechanical pressure to hydraulic force, then back to mechanical force at the wheel brakes. It uses the incompressibility of fluids. A 100 pound force on the pedal can push a 1 sq. in. piston to create 100 psi which pushes on 4 sq. in. output pistons to produce 400 pounds of force multiplying the driver's effort.
Dual Piston Master Cylinders:
A pushrod connects to a piston inside the cylinder with hydraulic fluid in front. When the pedal is pressed, the piston moves forward, transmitting force to all inner surfaces moving pistons in wheel cylinders or calipers outward to force shoes or pads against drums or rotors.
Master Cylinder Reservoir:
The reservoir may be cast with the cylinder body or separate nylon/plastic. One‑piece cast iron reservoirs have a single cover held by a retainer bail. Plastic reservoirs typically have two screw caps. Separate reservoirs may be clamped, bolted or pressed into holes with grommets. Caps are vented to prevent vacuum lock, a rubber diaphragm separates fluid from air while moving with fluid level changes preventing moisture and air entry. For front disc/rear drum systems split front rear, the disc brake reservoir chamber is larger because as disc pads wear, caliper pistons move out, requiring more fluid. Drum brake wheel cylinder pistons retract fully regardless of lining wear, so less fluid is needed. Vehicles with four wheel discs or diagonally split systems have equally sized reservoirs. Plastic reservoirs are often translucent for quick level checks but stains can give false indications and contamination cannot be seen without removing caps.
Master Cylinder Ports:
The forward port is the vent port and the rearward is the replenishing port per SAE J1153. The vent port has been called compensating, replenishing, bypass, filler or intake port. Both ports let fluid pass between each pressure chamber and reservoir during operation.
Master Cylinder Construction:
A single bore contains two piston assemblies: primary (rear) and secondary (front). Each has a return spring, cup seal in front and a seal at the rear. Inside are spool shaped pistons with a head and a groove for an O‑ring seal. The valley area allows fluid behind the piston head. Each piston has a rubber cup seal with flexible lips that seal pressure ahead. The cup lip bends to allow fluid from behind; when brakes are applied, pressure forces the lip against the cylinder wall to hold high pressure. The lip faces the pressure to be contained. Cup seals seal in one direction. Coil springs return pistons when pedal is released. Snap rings hold components, and a rubber boot keeps dirt out. A two piece master cylinder has an aluminum body with an anodized bore and a removable nylon/plastic reservoir, called a composite master cylinder. The pistons, cups and springs work the same as in one piece cylinders.
Master Cylinder Operation:
The vent port is located just ahead of the piston cup, allowing fluid from reservoir into the pressure chamber. The replenishing port is above the valley area. The O‑ring seal prevents leakage. When the pedal is depressed, the pushrod pushes the piston forward, covering the vent port, trapping fluid, which goes through outlet lines to apply brakes. When released, the return spring forces the piston back. As it moves back, it creates low pressure ahead, fluid flows from the valley area through holes in the piston head or around the cup into the chamber, relieving low pressure. The fluid from the system returns to the reservoir through the vent port when the piston is released, allowing brakes to release.
Residual Pressure Check Valve:
Some drum brake master cylinders have a residual pressure check valve in the pressure chamber or outlet line, maintaining 6 to 25 psi residual pressure when pedal is released. This keeps slight pressure on wheel cylinder pistons to hold sealing lips against walls, preventing air from being drawn in if pressure drops too low. It does not overcome brake shoe spring tension. Disc brake systems do not use this valve because it would cause pad drag. The valve may be under the tubing seat or inside the bore. When brakes are applied, pressure opens the valve, when released, line pressure unseats the valve to allow return flow, and when pressure drops below spring pressure, the valve closes to hold residual pressure. Many late‑model master cylinders have eliminated this valve, using piston cup expanders instead. Diagonally split systems also eliminate it because residual pressure would cause brake drag with disc brakes. Always verify if a vehicle requires this valve when replacing the master cylinder.
Split Hydraulic Systems:
Most late model vehicles have diagonally split systems. If the secondary piston circuit fails, both pistons move forward but only secondary spring resistance exists, primary builds little pressure until secondary bottoms, then primary builds pressure to operate its brakes. If the primary circuit fails, the primary piston moves forward without building pressure, little force transfers to the secondary until the extension screw contacts it, then pressure builds to operate secondary brakes.
Fast Fill and Quick Take Up Master Cylinders:
Some manufacturers use fast fill or quick take up master cylinders to fill the system quickly for low‑drag disc brakes which retract pistons farther from rotors to reduce drag and improve fuel economy. Conventional cylinders would require excessive pedal travel on first stroke. These cylinders have a larger diameter bore for the primary piston and a quick take up valve replacing conventional ports. They provide a large volume of fluid on the first stroke. The valve has a spring loaded check ball with a bypass groove. With brakes off, ports are open, fluid flows through the groove. When brakes are applied, the primary piston moves, and the larger primary chamber creates instant pressure that forces fluid past the cup seal to take up caliper slack. The lip seal prevents backflow, initially some fluid bypasses the ball, but pressure rises to 70–100 psi, opening the valve to let excess return. Pressure equalizes and the system acts normally. When released, return springs move pistons back, fluid bypasses seals, and fluid from reservoir flows through ports to equalize pressure.
Central Valve Master Cylinders:
Some ABS master cylinders have central check valves in pistons to prevent seal damage and pedal vibration during antilock operation. They allow fluid to flow through open valves when brakes are released, and close when applied. They provide supplementary passages for rapid fluid movement during ABS operation, reducing vibration and wear.
Hydraulic Tubes and Hoses:
Steel tubing and flexible synthetic rubber hoses transmit brake fluid from the master cylinder to wheel cylinders and calipers. Tubing is usually copper‑fused double‑wall steel, diameters from 1/8 to 3/8 inch. Some OEM tubing uses soft steel strips sheathed with copper, rolled into double wall and bonded at high temperatures with tin‑plating for corrosion protection. Fittings include double or inverted flare, the most common. Single flare or compression fittings may not hold up. Fittings are steel or brass, 37‑degree inverted flare is common. Newer vehicles may use ISO metric bubble flare fittings. Never change fitting style; use matching metal composition to avoid corrosion. Brake hoses offer flexible connections, ranging from 10 to 30 inches, made of multiple layers of fabric impregnated with synthetic rubber, offering high heat resistance.
Hydraulic System Safety Switches and Valves:
Pressure Differential Warning Light Switch:
This valve operates a warning light to inform the driver of pressure loss in either hydraulic circuit. Under normal conditions, pressure on both sides balances, piston centered, warning light off. If a leak occurs, pressure imbalance moves the piston, pushing the plunger to close the switch and illuminate the light. Three common design variations exist: with centering springs, without, and with two pistons.
Metering and Proportioning Valves:
These balance disc and drum brake characteristics. Disc brakes respond immediately, drum brakes are delayed due to return springs and self‑energizing action. A metering valve in the front line delays pressure to front disc calipers until rear drums build pressure improving balance and preventing front lockup especially during light braking. A proportioning valve controls rear brake pressures during hard stops to prevent rear lockup when pressure reaches a specified level, it stops flow to rear brakes, regulating pressure to keep front and rear forces balanced.
Height Sensing Proportional Valve:
This valve provides two brake balance modes based on vehicle load. When unloaded it reduces rear pressure, when loaded the actuator lever moves allowing full rear pressure. It contains a plunger, cam, torsional clutch spring and actuator shaft. Mounted above the rear axle, its lever connects to the lower shock absorber bracket. The torsional spring overrides position changes during bumps. These valves are not adjustable and replaced when defective.
Combination Valves:
Most newer cars have a combination valve that integrates metering, proportioning and pressure differential valves. Three function valves perform metering, warning light and proportioning. Two function variations combine proportioning with warning, or metering with warning. If any function fails, the entire valve must be replaced.
Warning Lights and Switches:
Failure Warning Lamp Switch:
It is the pressure differential valve described above. Each side connects to half the hydraulic system. Failure in one circuit causes pressure loss, moving the valve's plunger to close the circuit and illuminate the lamp. Most late model vehicles have this as part of a combination valve or built into the master cylinder. Some vehicles use a float switch in the reservoir instead.
Master Cylinder Fluid Level Switch:
Many vehicles have a fluid level switch that illuminates the red brake warning lamp when fluid is low. This can replace the pressure differential valve. Sensors are built into the reservoir body or cap. One type uses a float with contacts when the float drops, contacts close the circuit. Another uses a magnet in a float that pulls contacts together. Typically these provide a ground path for the lamp.
Parking Brake Switch:
If the parking brake is partially applied while moving, it generates heat that glazes friction materials, expands drums and increases pedal travel. On rear disc systems with integral parking brake actuators, driving with parking brake applied will distort rotors and reduce pad life. A normally closed switch in the parking brake handle or pedal assembly grounds the brake warning lamp circuit whenever the parking brake is applied. Some vehicles with daytime running lights use this switch to prevent headlights from coming on if parking brake is applied at start.
Stop Lamps:
They are in the tail lamp assemblies; vehicles since 1986 have a center high mounted stop lamp. Switches are hydraulically or mechanically operated. Hydraulic switches were used on older vehicles, activated by system pressure. Mechanical switches are mounted on the brake pedal bracket and activated by pedal movement, they are preferred because they can be adjusted to illuminate with slight pedal movement. Switches may be single function or multifunction with contacts for torque converter clutch, cruise control, or ABS. Brake lamp switch contacts often connect through the turn signal and hazard flasher switch. ABS vehicles have a connection or separate switch for the ABS control unit.
Drum and Disc Brake Assemblies:
Drum Brakes:
A drum brake assembly consists of a cast iron drum bolted to and rotating with the wheel and a fixed backing plate holding shoes, wheel cylinders, automatic adjusters, linkages and parking brake hardware. Shoes with frictional linings contact the inside of the drum when brakes are applied, forced outward by pistons in the wheel cylinder actuated by hydraulic pressure. The energy of the moving drum is transformed into heat and dissipated. When the pedal is released, hydraulic pressure drops and return springs pull pistons back.
Disc Brakes:
Theyhave pads that clamp against a rotor, a separate unit inboard of the wheel. The rotor is cast iron with both sides machined smooth often ventilated with a finned center section for cooling. Pads are attached to metal backings actuated by pistons in a caliper assembly that wraps around the rotor. The caliper is bolted to the suspension framework. It contains pistons, seals, springs, boots and fluid passages. Some models use spring pressure to keep pads close, others use a square cut seal that distorts during application and returns to retract the piston. Unlike drum brakes, disc pads act perpendicular to rotation and are non self energizing, requiring more force so they are usually used with power brakes.
Power Brakes:
Power brakes are a standard hydraulic system with a booster between the pedal and master cylinder to assist. Two types: vacuum assist using engine vacuum or external pump and hydraulic assist using power steering pump pressure on larger vehicles. Both multiply driver force, increasing hydraulic pressure and reducing pedal effort.
Vacuum Assist Power Brakes:
All vacuum assisted units generate application energy by opposing engine vacuum to atmospheric pressure using a piston, diaphragm or bellows. Modern units are vacuum suspended with the diaphragm balanced by vacuum until pedal is pressed, then atmospheric pressure unbalances it to generate pressure. Atmospheric pressure is 14–15 psi with a 12‑inch diaphragm, area is about 113 sq. in. Vacuum at about 7 psi creates a differential of 7.7 psi producing about 870 pounds of force. Boosters may be single or tandem diaphragm. The unit has three elements: a vacuum power section (shells, diaphragm, return spring, pushrod), a control valve (integral with diaphragm, connected to pedal) and a hydraulic master cylinder. Operation: when brakes are applied, the valve rod moves, closing the vacuum port and opening the atmospheric port, admitting air. Vacuum in the rear chamber creates force moving the diaphragm and pushrod to apply master cylinder pressure. As pressure develops, counterforce acts through the pushrod and reaction disc giving the driver feel.
Pushrod Adjustment:
Proper adjustment is necessary. Too long a pushrod closes the vent port causing brake drag. Too short causes excessive pedal travel and groaning. A properly adjusted pushrod from production requires no adjustment unless components are replaced. Two methods: gauge method using a gauge to check length and air method applying 5 psi compressed air to the master cylinder hydraulic outlet, if air passes through the replenishing port, adjustment is satisfactory, if not adjust the pushrod or add shims until air flows freely.
Hydraulic Brake Boosters:
Hydraulic assist systems use fluid pressure from the power steering pump mostly on diesel engines with low vacuum. The booster is between the cowl and master cylinder. Three hoses route fluid: supply from pump pressurized to steering gear and return to pump. Hydraulic pressure in the booster is separate from brake line pressure. Never mix fluids power steering fluid in the brake reservoir requires flushing. Some systems have a nitrogen charged accumulator for reserve assist. The booster consists of an open center spool valve, lever assembly, input rod, power piston, output pushrod and accumulator.
Operation:
When brake pedal is depressed, the lever moves a sleeve to close holes, then moves the spool valve to divert fluid behind the piston, building pressure that moves the piston and pushrod to apply master cylinder pressure. When released, spool and sleeve return and fluid returns. Common systems include Bendix hydro boost and GM Powermaster (which has its own vane pump and motor). Troubleshooting: inspect belt, fluid level, hoses. With engine off, pump pedal to bleed accumulator, hold pressure, start engine; pedal should move downward. Accumulator test: rotate steering wheel to lock for 5 seconds, return to center, shut off engine, pump pedal should get two to three power assisted strokes. Wait one hour, pump again should get same. If not, accumulator is leaking. Noise issues: moan or hum at low speeds due to low fluid or air, check fluid allow to sit. High speed fluid noise near pedal fully depressed is normal. A hiss when accumulator pressure is used is normal. A hiss after starting might be normal but if persistent without assist, accumulator may not hold pressure. Gulping after bleeding is normal. Powermaster testing requires special adapter and gauge.
Electric Parking Brakes:
Electrically operated parking brakes are becoming more common, replacing mechanical systems. They operate hydraulically for normal braking and electrically for parking, eliminating the parking brake lever or pedal, freeing up interior space. They are a first step toward brake by wire. Two techniques: electric motor mounted on rear calipers or undercar motor pulling cables. With caliper mounted motors, cables and linkages are eliminated. The motor is controlled by the PCM and interfaces with the CAN network for monitoring. Functions include: emergency braking control, helping stop if hydraulic system fails, automatic release when throttle opens, automatic engagement when ignition turned off or driver's door opens and hill hold assistance by applying rear brakes until clutch or throttle is operated.
0 Comments