If the booster unit has a bad vacuum leak it will make the engine run bad. The
system may leak due to bad hoses, leaking vacuum reserve tank, ruptured
diaphram, or possibly as you suggest, a bad "valve" somewhere in the unit.
It is easy enough to determine if a leaky booster unit is causing the engine to
run badly. This can be done by disconnecting the vacuum line at the back of the
intake manifold, and capping the vacuum source. Start the engine. If it smooths
out, then the booster (or some other part of the system) is at fault. If not,
then there is something else wrong.
Low compression on two cylinders wouldn't always cause the engine to run badly,
unless the difference is significant. Many old engines have some variation
between cylinders and still run well enough to do the job.
If there is a serious lack of compression, then the problem should be
diagnosed. There have been many good ideas put forth in other posts about what
could cause this.
A serious lack of compression can also cause a lack of vacuum that might effect
the way the brakes operate. I once had a car that developed a hole in the top
of a piston. This condition messed things up enough that, if I could make it
run, none of the vacuum operated accessories functioned properly.
I hope that this is helpful.
Paul
In a message dated 10/30/2003 11:06:34 AM Eastern Standard Time, "Hugh &
Therese" <hugtrees@xxxxxxxx> writes:
>I am delighted to take the correction with regards to vacuum. In fact, it
>makes my life a little easier for a different reason.
>
>However, my advice still stands, I think. Fixing something that is not the
>problem makes no sense when there are viable ways to find out what the
>problem really is. I maintain that problems on two cylinders only, numbers
>four and six, are unlikely to be caused by the booster.
>
>I have the same unit. My engine ran fine without any brakes when the
>booster unit failed because the vacuum tank was fine. The booster unit
>would not retract when the brakes were applied but vacuum in the tank was
>constant otherwise. If the tank is at fault, or the hoses, then it is not
>the booster unit. It could be a valve within the booster unit. Or, it's
>something else. If the engine cannot create sufficient vacuum in the tank,
>fixing the booster unit will take time, money, effort and disruption and
>achieve nothing.
>
>Hugh
>
>
>
>
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