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Re: brake tests

frank a sez:
>>>Are there any other tests for the bomb, besides the engine off pump the
>>>brakes test?
>>this is the only good way to test it.  why do you want another test?
>Because I have an intermittent problem where: the brake light comes on,
>pedal pulsates, zero power assist. Then all of a sudden, maybe at the next
>startup or two or three later, the brake light goes off, pedal no longer
>pulsates, power assist is back, and here is the killer, if I do the pump
>test at the next shut down I get 27 to 34 pumps before I lose power assist.
> ...

i thought it might be this type of failure.
here's my stab (guess) at a possible explanation.
there are two valves built into the pressure accumulator.
the check valve bleeds excess fluid when operating pressure is reached
(2030 psi).  the pressure relief valve releases pressure (@ 2175psi) if
the check valve sticks shut.
(with a positive displacement pump, you need this safety release or you
would truly have a bomb in the engine compartment.)
possibly one of these two valves is intermittently opening.
more likely the first one i described is sticking.
whenever it decides to snap shut, things work as they should.
the only way to test these valves is to remove the pressure switch from
the hydraulic servo (which is attached to the master cylinder) and
attach a pressure gauge with a hollow bolt to observe the hydraulic
pressures during operation.

after thinking about what i just wrote, i realized it could also be the
hydraulic servo working intermittently.  if this sticks open, it would
constantly demand pressure from the bomb, possibly enough to prevent any
pressure buildup.  you can test this by removing the return line from
the servo (to the reservoir) to see if fluid is flowing (a few drops are
ok) without any brake application.

other thoughts, anyone?

frank b

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