3.2 Diagnostic Tests

Some general checks of brake system function and component inspection can help isolate problems.

Test the rear brakes by applying the parking brake. It should be able to hold the car, perhaps even stalling the engine, when you try to drive away. If not, the brakes probably need adjustment or reconditioning.

Test the master cylinder by holding the pedal down hard with the car stopped and the engine running. The pedal should feel solid and stay solid. If the pedal slowly falls to the floor and there is no evidence of fluid leaks in the system, then the master cylinder is faulty and should be replaced.

On cars without ABS, test the vacuum booster by first pumping the brake pedal a few times with the engine off, and then holding the pedal down. When the engine is started, the pedal should fall slightly. The booster diaphragm rarely fails, but leaks in the vacuum line or a faulty check valve will decrease the vacuum assist. Check the valve by removing it from the booster vacuum line and blowing through it. Air should pass through in the direction of the arrow on the check valve, but not through the opposite way. On cars with diesel engines, vacuum booster problems may be caused by a faulty vacuum pump. If brake fluid is leaking into the booster from the master cylinder, the master cylinder is defective and should be replaced.

On ABS-equipped cars, quick-test the ABS hydraulic pump and pressure warning switch as follows. With the engine not running and the ignition off, pump the brake pedal 25 to 35 times to dissipate the residual pressure in the ABS hydraulic system. Then turn the ignition on. The hydraulic pump should start and run for about 60 seconds, then stop. Depress the pedal again two or three times; the pump should run again briefly. If at this time the ABS warning light stays lit, there is a fault in the system and additional troubleshooting is required.