Brake System Testing

Brakes should be tested on a dry, clean, reasonably smooth and level roadway. A true test of brake performance cannot be made if roadway is wet, greasy, or covered with loose dirt whereby all tires do not grip road equally. Testing will also be adversely affected if roadway is crowned so as to throw weight so roughly that wheels tend to bounce.

Test brakes at different vehicle speeds with both light and heavy pedal pressure: however, avoid locking brakes and sliding tires. Locked brakes and sliding tires do not indicate brake efficiency since heavily braked, but turning, wheels will stop vehicle in less distance than locked brakes. More tire-to-road friction is present with a heavily-braked turning tire than with a sliding tire. Because of high deceleration capability, a firmer pedal may be felt at higher deceleration levels.

There are three major external conditions that affect brake performance:

• Tires having unequal contact and grip of road will cause unequal braking. Tires must be equally inflated, and tread pattern of right and left tires must be approximately equal.
• Unequal loading of vehicle can affect brake performance since most heavily loaded wheels require more braking power, and thus more braking effort than others.
• Misalignment of wheels, particularly conditions of excessive camber and caster, will cause brakes to pull to one side.

  To check for brake fluid leaks, hold constant foot pressure on pedal with engine running at idle and shift lever in NEUTRAL. If pedal gradually falls away with constant pressure, hydraulic system may be leaking. Perform a visual check to confirm any suspected leaks.

  Check master cylinder fluid level. While a slight drop in reservoir level results from normal lining wear, an abnormally low level indicates a leak in system. The hydraulic system may be leaking either internally or externally. Also, system may appear to pass this test while still having a slight leak. If fluid level is normal, check vacuum booster pushrod length. If an incorrect pushrod length is found, adjust or replace rod.

  Check master cylinder using following procedure:

• Check for a cracked master cylinder casting or brake fluid leaking around master cylinder. Leaks are indicated only if there is at least one drop of fluid. A damp condition is not abnormal.
• Check for a binding pedal linkage and for an incorrect pushrod length. If both of these parts are in satisfactory condition, disassemble master cylinder and check for an elongated or swollen primary cylinder or piston seals. If swollen seals are found, substandard or contaminated brake fluid should be suspected. If contaminated brake fluid is found, all components should be disassembled and cleaned, and all rubber components should be replaced. All of pipes must also be flushed.

Improper brake fluid, or mineral oil or water in fluid, may cause brake fluid to boil or cause deterioration of rubber components. If primary piston cups in master cylinder are swollen, then rubber parts have deteriorated. This deterioration may also be evidenced by swollen wheel cylinder piston seals on drum brake wheels.

If deterioration of rubber is evident, disassemble all hydraulic parts and wash parts with alcohol. Dry these parts with compressed air before reassembly to keep alcohol out of system. Replace all rubber parts in system, including hoses. Also, when working on brake mechanisms, check for fluid on linings. If excessive fluid is found, replace linings.

If master cylinder piston seals are in satisfactory condition, check for leaks or excessive heat conditions. If these conditions are not found, drain fluid, flush master cylinder with brake fluid, refill master cylinder, and bleed system.