it's possible to have a vacuum leak between engine and booster..
and a check valve issue as noted.

there is a classic test..
which you can even look up online I'm sure.

I think it goes like this ..
with engine off...
apply brakes several times  until all vacuum is  gone from the booster
system ..

then, while holding down the brake pedal, start the engine.
the pedal should immediately sink an inch or so.

in a rare case I have measured the vacuum with a gauge that's getting to
the booster.
on my own first vanagon
it had 'no brake boost affect' after getting it jumped started one cold
winter day on my first big trip in it..

somehow ...never did figure out how or why ..
there was a small hole on the underside of the hard plastic line coming
off the vac pump ( diesel vanagon )
like someone had heated the tip of an ice pick and melted a small hole
in the underside of that line by the engine.
Total mystery how that happened , but that was it.

rare for a booster to actually be bad.

On 9/18/2014 2:18 PM, Dennis Haynes wrote:
> What are the symptoms? Rarely do these go bad but it does happen. Often the check valve goes bad which is an easy repair. A bad engine especially an intake valve can result in poor brake booster operation. How is the mechanic testing it?
>
> Dennis
>
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers
> Sent: Thursday, September 18, 2014 7:52 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Brake Booster - 1991 Carat
>
> My brake booster is dead - or so says the mechanic. A boosteris near $450 and List Vendors only sell new ones.
>
> I'm not convinced its leaking but don't knw enough about them to know. I do know its a big job to change one out.
>
> Is there any thing that should be checked before undertaking this? I really hate this.
>
> Mechanic says it has a vacuum leak.
>
> Thanks.
>
> John
>