That o-ring comes to mind of course.
I suggest removing the instrument cluster..
start engine, apply brakes hard, and listen, with a listening tube held to
your ear, for a hiss from the base of the master cylinder, where it's bolted
to the booster.

also check the tightness of those nuts ..
just in case someone didn't tighten them very well.
Usually they are quite tight.....and easy to check, and it fits the symptom
.

you don't have to completely remove the master cylinder to check the o-ring
..
you can just undo the two nuts and push the MC back a qtr inch or so to peak
in there.

Scott
www.turbovans.com
----- Original Message -----
From: "Ken Wilford" <kenwilfy@COMCAST.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Monday, October 31, 2011 4:50 AM
Subject: Re: brake hissing?

> Wes, it could just be the o-ring between the master cylinder and the
> booster.  I have had customers come in and we check it out and that
> o-ring has just disintegrated.  Of course it could also bee the booster
> itself.  You really aren't going to know for sure until you pull the
> master cylinder off to check the o-ring.  Let me know if I can help you
> further.
>
> Ken Wilford
> John 3:16
> www.vanagain.com
>
>
> On 10/30/2011 11:42 PM, pickle vanagon wrote:
>> I've noticed that with hard brake application, I slight hissing sound can
>> be heard coming from under the dash in our 83 westy.  I fear it's the
>> brake
>> booster?
>>
>> You don't need to be moving to hear it.  When idling, if I push in the
>> brake medium-far, I hear nothing, but pushing down *hard*, I hear the
>> hiss.   It seems like the noise is on or off, and doesn't vary
>> continuously
>> with depth.
>>
>> There is no fluid leak....
>>
>> Is this a problem?  I don't usually push down the brakes hard, so I'm
>> wondering if its down this for a while without me noticing.
>>
>> I'm afraid to ask the next one...
>> do I need to remove the dash to replace the booster?
>>
>> Thanks!
>> Wes
>>