Power Brake booster - How to tell its bad?

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Old 11-09-05, 03:24 PM
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Power Brake booster - How to tell its bad?

How do you tell when a power brake booster is bad?

My problem is with a 1989 Toyota Celica, ST. I've replaced the front calipers, front pads, front rotors, adjusted the rear pads, as well as bled the brakes. The pedal feels firm with the engine off, but when the engine is on, the pedal seems to go to further down than it should before it stops the car. The pedal does NOT go to the floor and if I slam on the brakes, I can easily make the wheels lock and stop the car. It just seems to travel farther down than it should.

I ask because I saw a prior question about a soft pedal that mentioned a bad brake booster. I have also noticed that if I am at idle, sitting still and hit the brakes all of a sudden the tach will show the engine RPM go way up for a second - so I am thinking there might be a vacuum leak in either the booster hose or the booster itself. However, I always thought if there was a bad booster, it would just make it very hard to brake - which this isn't.

Please help & thanks
 
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Old 11-09-05, 05:33 PM
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Bad booster doesn't give you a soft pedal, like you said just no assist (even that usually a vac supply problem)

To test with engine off pump pedal after a few strokes it should get high and hard. Then while holding the pedal down start engine and pedal will fall away (assist). If as above its ok.

Clamp each brake hose at the wheel and test pedal, when pedal is good thats the wheel that has the problem. Causes: hanging caliper slide, rusty shims or bad brake hose.
 
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