Date: Mon, 27 Feb 1995 22:31:45 CST6CDT
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: "Dan Houg" <fairwind@northernnet.com>
Subject: Brake booster troubleshooting
I have plaguarized the following information on diagnosing brake
booster problems. It is from Automotive suspension, steering and
brakes by Ellinger and Hathaway.
Vacuum-assist problems can be diagnosed by following a logical
sequence of steps. The first step is to make sure that the unit is
receving the proper vacuum. A restricted and/or cracked hose would
result in little or no assist and after repeated brake applications,
the pedal would become more diffficult to depress.
Another test is to check for vacuum leaks on the power booster itself and for faulty
operation of the check valve. With the engine turned off, listen
for a vacuum leak. If no vacuum leaks are heard, wait two or three
minutes, then depress the brake pedal to see if vacuum is still
present in the reserve system. A leak is present if there is no
assist on the first brake application. The leak will be through
either the check valve or an external leak. To test for a
malfunction of the check valve, close the vacuum hose off after the
egine has idled to make sure full vacuum is in the system. The
above steps are then repeated. If vacuum is held in the unit with
the hose closed off, the check valve is faulty.
A check for assist can be made by first depressing the pedal several
times with the engine off to make sure that no vacuum is in the system.
The brake pedal is then held down and the engine is started. This
should cause the pedal to 'give' and move down slightly.
If the unit does not leak during the reserve check it is possible the
problem then exists inside the booster unit. With the brake pedal
depressed and the engine running, listen for a hissing sound caused
by vacuum leakage. If it cannot be heard, stop the engine and
continue listening. A steady hissing sound with the pedal depressed
that goes away with the pedal released indicates a diaphragm or
poppet valve (a thingy inside the booster) malfunction. replacement
of the booster is required to correct this problem.
-dan