Expert: Sean Halliday - 11/24/2008

Question
QUESTION: I am having a problem with the brakes on a 1995 GM Commodore. The brake pedal intermittently goes down .25 inch
when the engine is started and this means the brake is slightly on and a small loss of power is felt. If I push the brake pedal as the car is started the pedal goes down
as it should with power assisted brakes and it fixes the problem until the next time the car is used and only if I forget to push the pedal. The booster is holding its vacuum overnight and can be heard
when pumping the pedal a few times without the engine running.I have compared the vacuum in the hose to the booster of 2 similar cars mine is 470mmHg and other car 440mmHg. Do you think the booster is faulty or would the extra vacuum cause the problem even though it is not significantly higher.


ANSWER: So what exactly is wrong with the brakes?Do they not stop the car or fade or what?Did you check the fluid first for leaks?And the wheel cylinders and calipers?

---------- FOLLOW-UP ----------

QUESTION: The brakes do work ok ,but there is a slight loss of power
because the pedal has been pulled slightly down when the engine is started, I am thinking that the return spring in the booster is a little week and the vacuum on start up is allowing the pedal to go down slightly.Do you know if this a symptom of a faulty booster?

Answer
A faulty booster will not hold the vacuum and is easily tested by driving and applying the brakes quickly in succession(find a safe stretch of road) and if the pedal goes hard then its faulty.Also if the pedal goes really soft then have it replaced.The best way is to have the booster tested by the local dealer with their diagnostic machine designed especially for this,may be worth it if the booter fails ,you may run out of brakes when you need them.