Expert: Van - 8/27/2005

Question
We have an intermittent "hard" brake pedal.  It usually happens at higher speeds and/or with the air conditioner on.  The pedal is very stiff, and you have to pump the pedal hard to get it to go down and stop the vehicle.  Before the brakes kick in, the ABS sounds like it is engaged.  This doesn't happen every time the vehicle is driven and is completely unpredictable.  We have recently replaced/repaired both the front and rear brakes, and the EGR valve was cleaned in April, 2005.  All fluid levels are normal, and a rebuilt master cylinder was installed approximately 5 years ago.  Have you ever heard of anything like this, and if so, what can we do to eliminate the problem?

Answer
Hi Bob,
Sounds like the brake booster isn't working during those times.
it operates on vacuum.
The a/c heater controls also work on vacuum.
I am thinking there may be a leak in the control system, although it would be a large one to cause that kind of problem with the brake booster, so it should also make the engine run rough.

It could be the brake booster that is bad, but the comment about only when the a/c is on seems significant.
I would try to isolate it some more with the a/c controls, and probably even connect a vacuum gauge where you can see it while operating the vehicle.
The abs may be working just as designed.
Van