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Brakes, Page 2 of 3

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Drum brakes
See Figures 6 and 7

Drum brakes use two brake shoes mounted on a stationary backing plate on each wheel. These shoes are positioned inside a circular cast iron drum that rotates with the wheel assembly. The shoes are held in place by springs; this allows them to slide toward the drums (when they are applied) while keeping the linings and drums in alignment.

The shoes are actuated by a wheel cylinder that is usually mounted at the top of the backing plate. When the brakes are applied, hydraulic pressure forces the wheel cylinder's two actuating links outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are then forced outward against the inner side of the drum. This action forces the bottoms of the two shoes to contact the brake drum by rotating the entire assembly slightly (known as servo action). When pressure within the wheel cylinder is relieved, return springs pull the shoes away from the drum.

Modern drum brakes are designed to self-adjust during application when the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever. The self-adjusters are only intended to compensate for normal wear. Although the adjustment is "automatic," there is a definite method to actuate the self-adjuster, which is done during normal driving. Driving the vehicle in reverse and applying the brakes usually activates the automatic adjusters. If the brake pedal was low, you should be able to feel an increase in the height of the brake pedal.

Figure 6 Exploded view of a typical drum brake assembly.
Click on picture to enlarge view

Figure 7 Common dual return spring drum brake setup component identification.
Click on picture to enlarge view

Disc brakes
See Figures 8, 9 and 10

Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems utilize a cast iron rotor (disc) with brake pads positioned on either side of it. Braking effect is achieved in a manner similar to the way you would squeeze a spinning disc between your fingers.

The rotor (disc) is a one-piece casting with cooling fins between the two braking surfaces. This enables air to circulate between the braking surfaces making them less sensitive to heat buildup and more resistant to fade. Dirt and water do not affect braking action since contaminants are thrown off by the centrifugal action of the rotor (disc) or scraped off by the pads. In addition, the equal clamping action of the two brake pads tends to ensure uniform, straight-line stops. All disc brakes are inherently self-adjusting.

There are three general types of disc brake:

  • A fixed caliper
  • A floating caliper
  • A sliding caliper

The fixed caliper design uses one or two pistons mounted on each side of the rotor (in each side of the caliper). The caliper is mounted rigidly and does not move.

The sliding and floating designs are quite similar. In fact, these two types are often lumped together. In both designs, the pad on the inside of the rotor is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly, bringing the outside pad into contact with the rotor.

Floating calipers use threaded guide pins and bushings, or sleeves to allow the caliper to slide and apply the brake pads.

There are typically three methods of securing a sliding caliper to its mounting bracket: with a retaining pin, with a key and bolt, or with a wedge and pin. On calipers that use the retaining pin method, you will find pins driven into the slot between the caliper and the caliper mount. On calipers which use the bolt and key method, a key is used between the caliper and the mounting bracket to allow the caliper to slide. The key is held in position by a lock bolt. On calipers which use the pin and wedge method, a wedge, retained by a pin, is used between the caliper and the mounting bracket.

For pad removal purposes, fixed calipers are usually not removed, floating calipers are either removed or flipped (hinged up or down on one pin), and sliding calipers are removed.

Figure 8 Typical disc brake components.
Click on picture to enlarge view

Figure 9 Common front disc brake component identification.
Common front disc brake component identification.
1. Brake caliper
2. Brake hose
3. Support (anchor) plate
4. Outboard brake pad
5. Rotor
6. Rotor retainer

Figure 10 Typical rear disc brake component identification.
Typical rear disc brake component identification.
1. Brake caliper
2. Support bracket
3. Outboard brake pad
4. Rotor retainer
5. Rotor

Power brake boosters
See Figure 11

Power brakes operate just as standard brake systems, except in the actuation of the master cylinder pistons. A vacuum diaphragm is located behind the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.

The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve at the point where the hose enters the diaphragm housing ensures that during periods of low manifold vacuum brake assist vacuum will not be lost.

Depressing the brake pedal closes the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm, and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder-actuating rod and direct mechanical application will occur as the pedal is depressed.

The hydraulic and mechanical problems that apply to conventional brake systems also apply to power brakes.

Figure 11 Exploded view of a typical vacuum booster assembly.
Click on picture to enlarge view

Parking brake
See Figures 12, 13 and 14

The emergency or parking brake is used simply to hold the vehicle stationary while parked. It has no hydraulic connection and is simply a means of activating the rear (usually) or front (rarely) wheel brakes with a cable attached to a floor-mounted lever or dash-mounted pedal or lever.

Figure 12 The parking brake linkage normally operates the rear brakes. Depressing the pedal (not shown) or pulling up on the lever expands the rear brake shoes against the drum.
Click on picture to enlarge view

Figure 13 Some models with rear disc brakes are equipped with a drum in hat style parking brake assembly.
Some models with rear disc brakes are equipped with a drum in hat style parking brake assembly.

Figure 14 Exploded view of a drum in hat style parking brake assembly-on this type of parking brake, there are brake shoes mounted inside the disc rotor.
Click on picture to enlarge view

Anti-lock brakes
See Figures 15, 16 and 17

Anti-lock Braking Systems (ABS) are designed to prevent locked-wheel skidding during hard braking or during braking on slippery surfaces. The front wheels of a vehicle cannot apply steering force if they are locked and sliding; the vehicle will continue in its previous direction of travel. The four wheel anti-lock brake systems found on many of today's vehicles hold the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways while braking. The Rear Wheel Anti-Lock (RWAL) systems used primarily on trucks and vans is designed to prevent the rear wheels from locking up during severe braking. Especially since these vehicles are often designed to carry heavy loads, the rear brakes can be very touchy when the truck or van is unloaded. RWAL systems usually utilize a load sensing mechanism to adjust the sensitivity of the system to compensate for heavy or no load situations.

There are conditions for which the ABS system provides no benefit. Debris, gravel, snow or sheets of ice render the ABS system ineffective since it relies on an underlying amount of road traction, which is not available when driving on gravel, excessive debris, snow or ice. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. This renders the vehicle totally uncontrollable until road contact is regained. Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking. For this reason, the system is named anti-lock rather than anti-skid.

Under normal braking conditions, the ABS system functions in the same manner as a standard brake system. The system is a combination of electrical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary.

The anti-lock brake system's Electronic Control Unit (ECU) is the electronic brain of the system, receiving and interpreting speed signals from the speed sensors. The ECU will enter anti-lock mode when it senses impending wheel lock at any wheel and immediately control the brake line pressure(s) to the affected wheel(s). The hydraulic actuator assembly is separate from the master cylinder and booster. It contains the wheel circuit valves used to control the brake fluid pressure to each wheel circuit. If the ABS becomes inoperative for any reason, the fail-safe system insures that the normal braking system is operative. The dashboard warning lamp is activated to show that the ABS is disabled.

Figure 15 Hydraulic line and wiring schematic for a common 4-wheel ABS system.
Click on picture to enlarge view

Figure 16 Identification of typical ABS system components-not all systems utilize these components.
Click on picture to enlarge view

Figure 17 Typical rear wheel only (RWAL) ABS system components.
Typical rear wheel only (RWAL) ABS system components.

To front brakes:

  1. Master cylinder
  2. Brake light Switch
  3. Instrument cluster
  4. Digital radio adapter (part of instrument cluster)
  5. Speed sensor
  6. Transmission
  7. Isolation/dump valve
  8. RWAL control module
  9. Brake warning light
  10. Combination valve

Typical system operation

A typical 4-wheel anti-lock brake system uses a 4-sensor, 4-channel system. A speed signal for each wheel is generated by a speed sensor at the wheel. The hydraulic actuator contains 4 control solenoids, one for each wheel brake line. On RWAL systems, there is either one wheel speed sensor mounted at each rear wheel or one sensor mounted in the differential case, which reads the axle speed. The hydraulic actuator controls the brake line(s) feeding the rear wheel brakes.

The system is capable of controlling brake line fluid pressure to any or all of the wheels as the situation demands. When the ECU receives signals showing one or more wheels about to lock, it sends an electrical signal to the solenoid valve(s) within the actuator to release the brake pressure the line. The solenoid moves to a position which holds the present line pressure without allowing it to increase. If wheel deceleration is still outside the pre-programmed values, the solenoid is momentarily moved to a position which releases pressure from the line. As the wheel unlocks or rolls faster, the ECU senses the increase and signals the solenoid to open, allowing the brake pedal to increase line pressure.

This cycling occurs several times per second when ABS is engaged. In this fashion, the wheels are kept just below the point of lock-up and control is maintained. When the hard braking ends, the ECU resets the solenoids to its normal or build mode. Brake line fluid pressures are then increased or modulated directly by pressure on the brake pedal. Fluid released to the ABS reservoirs is returned to the master cylinder by the pump and motor within the actuator.

On 4-wheel systems, the front and rear wheels are controlled individually, although the logic system in the ECU reacts only to the lowest rear wheel speed signal. This method is called Select Low and serves to prevent the rear wheels from getting greatly dissimilar signals which could upset directional stability.

The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during normal operation. The sounds are due to normal operation and are not indicative of a system problem. Under most conditions, the sounds are only faintly audible. If ABS is engaged, the operator may notice some pulsation in the body of the vehicle during a hard stop; this is generally due to suspension shudder as the brake pressures are altered rapidly and the forces transfer to the vehicle. There may also be a noticeable pulsation in the brake pedal as the hydraulic fluid is controlled by the ABS system; this is normal and should not be thought of as a defect in the system.

Although the ABS system prevents wheel lock-up under hard braking, as brake pressure increases wheel slip is allowed to increase as well. This slip will result in some tire chirp during ABS operation. The sound should not be interpreted as lock-up, but rather as an indication of the system holding the wheel(s) just outside the point of lock-up. Additionally, the final few feet of an ABS-engaged stop may be completed with the wheels locked; the electronic controls do not operate below about 3 mph (5 km/h).

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©1998 W. G. Nichols - Chilton's Easy Car Care