Disc brakes, Drum brakes, Free fit brakes


INFORMATION

Disc brake systems for cars and light trucks have been standard equipment for over a decade. By design, all disc brakes are self adjusting and therefore, they require no periodic readjusting to compensate for brake shoe wear. However, certain malfunctions may cause a disc brake to drag. This drag will reduce mpg and result in premature brake pad and rotor wear. Disc brakes are used primarily on front wheels but are also used on the rear of some vehicles. It is critical for both safety and maximum fuel economy that disc brakes operate properly! It is important to note that any brake system on a given vehicle operates in the proper front to rear sequence and that the left and right sides operate in unison to avoid undue premature lockup or lateral forces.
Figure 1

Figure 1 illustrates a typical disc brake. Adjustment is automatic. What causes disc brakes to drag? Several malfunctions are possible. On the floating caliper, single piston design, the most likely culprits are a build-up of dirt, corrosion, or loss of the protective lubrication on the guide pins, sleeves or ways. These cause the caliper to not fully retract when the brake pedal is released. Figure 2a and 2b illustrate a typical single piston floating caliper disc brake at rest and when activated.

Figure 2a.

Figure 2b.

Cross sectional view of single-piston caliper. Note that the hydraulic pressure pushes the piston outward and pulls the cylinder inward.

The four piston, non-floating design is used only on larger pick-up trucks and vans (see Figure 3). Four piston types tend not to retract as a result of corrosion build up in the caliper hydraulic. Small incremental increases in corrosion inside the caliper bores prevent the pistons from fully retracting after each brake application. If one or more pistons fail to fully retract, the shoe will drag the rotor resulting in excessive drag and reduced mpg.

Figure 3.

1. Caliper Assembly
2. Outboard Piston Housing (Caliper Half or Caliper Casting)
3. Inboard Piston Housing (Caliper Half or Caliper Casting)
4. Internal Fluid Passages
5. Piston Assembly
6. Piston Seal
7. Piston Boot (Dust Boot)
8. Shoe and Lining Assembly
9. Disc Brake Rotor

How are these problems corrected? If external corrosion, dirt, or lack of lubrication is the problem, disassemble in accordance with manufacturer's shop manual procedures. Clean parts with appropriate solvents. Inspect for wear, metal fatigue, and failure. Lubricate using the recommended lubricant and reassemble. If brake drag results from a piston or pistons not fully retracting, then caliper disassembly, inspection, and rebuilding will be necessary. Observe manufacturer's cautions in the shop manual. Note the recommended bleed down procedures for hydraulic systems in antilock braking systems and hydroboost type systems.

SUMMARY

Disc brake system failures are infrequent. Disc brakes can and do drag. Obvious indications of disc brake drag are:
1. Excessive wear of one brake pad (Note: it is normal for the inner pad of a single piston, floating type disc brake system to have a slightly higher wear rate than the outer pad.)
2. Vehicle pull. If all alignments are within specification, a car or truck can still pull to one side if a disc brake drags.
3. Tapered pad wear. This failure, most common on four piston, fixed caliper type, is caused by one or more pistons failing to fully retract.

ACTIVITY

WARNING: ALWAYS CONSULT THE MANUFACTURER'S SHOP MANUAL FOR EXACT BRAKE ADJUSTMENT PROCEDURES.
CAUTION: ALL BRAKE WORK HAS THE POTENTIAL FOR ASBESTOS HAZARDS. USE THE CORRECT EQUIPMENT AND PROCEDURES.
1. Locate an appropriate manual to determine correct inspection procedure.
2. Block the wheels so the vehicle will not roll. Release the parking brake and place transmission in neutral.
3. Raise the axle and place a safety stand at an appropriate place to support the weight of the vehicle.
4. By hand, try to rotate the wheel and tire assembly. If it does not move freely, remove the tyre or wheel assembly and proceed to check disc brake.
Using the shop manual as a guide, identify the type of disc brake and check for:
A. Excessive pad wear,
B. Tapered pad wire, or
C. Build up of dirt, corrosion, or lack of lubricant.
If any of these are noted, repair according to manufacturer's manual.
If the wheel and tire assembly did turn freely, the disc brake is retracting correctly.

An excessively tight disc brake causes vehicle pull, premature brake wear, and loss of mpg. A result is pad drag and reduced mpg. Even slight drag can reduce mpg. A non- releasing pad will not only wear out faster but will reduce the vehicle's mpg. It is important that students realize that even though disc brakes are virtually trouble free, over a period of time, lubricants get contaminated, are washed away, or dry out. The result is a brake pad that drags.