> Subject:       brake booster diagnostics
> 
> I'd posted this earlier but it seems pertinent again...
> 
> 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 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
> 
>