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Jim Grant's Tech Tips
96 GMC
Jimmy, ABS / Hissing Brake Pedal
Q: My '96 Jimmy has been in for the first
ABS recall and since that time, the brakes have gradually become less
effective. When applying brake pedal, a hissing sound comes from the area
of the pedal. If I pump the brakes once, all brake function disappears.
I have no ability to stop at all. The vehicle is scheduled for the second
recall in a few days and I want to make sure that I won't be misled into
replacing other brake parts. The 2nd recall involves replacing a vacuum
switch on the transfer case, and the hissing in the pedal certainly sounds
like a vacuum leak. What would cause the hissing, and also what would
cause the brakes to fail altogether after pumping just once?.....Rich
A: The
problem you’re describing and the recall for the brakes are not likely
related. Why? One recall is for a change in the Anti-lock Brake computer’s
program. The other involves a switch that tells the Anti-lock Brake computer
when the vehicle is in 4 or 2 wheel drive. Your condition seems related
to the power brake booster. The power brake booster makes use of engine
vacuum to provide power assist when braking. If the vacuum supply to the
brake booster is low the effort required to stop the vehicle greatly increases.
I’m talking the kind of push that causes your leg muscles to spaz just
to stop the vehicle. Most brake boosters have a vacuum reserve that is
good for 2 to 3 brake applications; anything less there’s a problem. If
the engine is not running correctly the amount of available vacuum to
the power brake booster is reduced, which reduces braking ability. This
can be checked easily with a vacuum gauge. If vacuum supply is low then
there’s a problem with the engine. If the vacuum supply is good then the
vacuum hose, vacuum check valve, or brake booster has a problem. From
your description you have a basic brake function problem that the recalls
are not likely to correct. Be sure to check the basics of the power brake
system first.
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