Whatislove

How do I test if I have a leaky/bad brake booster? My brake pedal has recently been going from stiff to soft. I dont have any leaks, master is fine and I just did a complete flush and bleed thinking it was some air.

It Wasn't Me

Functional Test

1. With the engine off, press the brake pedal several times to deplete the vacuum reservoir, then firmly press-and-hold the brake pedal for 15 seconds. If the brake pedal sinks, either the MC is internally bypassing, or there is a leak in the brake system. (mc, lines, prop valve, calipers, wheel cylinders, etc)

2. With the brake pedal pressed, start the engine. If the pedal sinks slightly, the vacuum booster is okay. However, if the brake pedal height does not vary, the check valve or booster is faulty.

3. With the engine running, lightly press the brake pedal. If the pedal sinks more than 3/8" (10mm) in three minutes, the master cylinder is faulty. (If the A/C is on, a slight change in the height of the brake pedal is normal.)





Leak Test

1. With the engine running, press-and-hold the brake pedal, then turn the engine off. If the pedal height does not vary while pressed for 30 seconds, the vacuum booster is okay. If the brake pedal rises, the booster is bad.

2. Turn the engine off and wait for about 30 seconds. Press the brake pedal several times using normal pressure. When the brake pedal is first pressed, it should be low, then gradually rise with each consecutive compression. If the position of the pedal does not vary, inspect the booster check valve.

3. Disconnect the vacuum hose (check valve built-in) at the brake booster.

4. Start the engine and let it idle. There should be vacuum available. If there is no vacuum, the check valve is not functioning properly. Replace the vacuum hose, and retest.

5. With the engine running, pinch the vacuum hose between the check valve and the booster.

6. Turn the engine off and wait for about 30 seconds. Press the brake pedal several times using normal pressure. Again, the brake pedal should be low when it's first pressed, gradually rising with each consecutive compression. If the pedal position does not vary, replace the booster. If the pedal position does vary, replace the vacuum hose / check valve assembly.