Brakes & ABS


Brake Booster
Functional Test
1. With the engine stopped, depress the brake pedal
several times to deplete the vacuum reservoir, then
depress the pedal hard and hold it for 1 5 seconds.
If the pedal sinks, either the master cylinder is
bypassing internally, or the brake system (master
cylinder, lines, modulator, proportioning valve, or
calipers) are leaking.
2. Start the engine with the pedal depressed. If the pe-
dal sinks slightly, the vacuum booster is operating
normally, if the pedal height does not vary, the
booster or check valve is faulty.
3. With the engine running, depress the brake pedal
lightly. Apply just enough pressure to hold back au-
tomatic transmission creep. If the brake pedal sinks
more than 25 mm (1.0 in.) in three minutes, the
master cylinder is faulty. A slight change in pedal
height when the A/C compressor cycles on and off
if normal. (The A/C compressor load changes the
vacuum available to the booster.)
Leak Test
1. Depress the brake pedal with the engine running,
then stop the engine. If the pedal height does not
vary while depressed for 30 seconds, the vacuum
booster is OK. If the pedal rises, the booster is faulty.
2. With the engine stopped, depress the brake pedal
several times using normal pressure. When the pe-
dal is first depressed, it should be low. On consecu-
tive applications, the pedal height should gradually
rise. If the pedal position does not vary, check the
booster check valve.
Booster Check Valve Test
1. Disconnect the brake booster vacuum hose at the
booster or at the booster side of the valve.
2. Start the engine and let it idle. There should be vacu-
um. If no vacuum is available, the check valve is not
working properly. Replace the check valve and