[Vwdiesel] brake servo

[James Hansen]

Actually, if you hear hissing, isn't the vacuum diaphragm in the brake
booster shot, or the vacuum control valve Loren?
Shouldn't hear hissing unless vacuum is escaping... well, not escaping
exactly, being rendered vacuumless or becoming less vacuumy would be more
correct...
But I never hear one inhale when brakes are applied. I suspect that's
normal.
Hissing or not would not be affected by the vacuum pump condition, but if
vacuum leaks down when you've applied vacuum, it indicates a leaky vacuum
control in the servo- that leaks to the area outside the valve, which
inhales from the vehicle interior when Bob moves the brake plunger, letting
it hiss to the vehicle interior. Test by partially applying the brake pedal
with a clamp or some fixture that does not move. Apply vacuum in the same
way you did to test leakdown, (isolating the servo, but allowing air to the
atmosphere side of the diaphragm), and vacuum should not go away.  If your
vacuum cannot be maintained, the booster is leaking somewhere internally.
-james
 
-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com] On
Behalf Of lbaird119 at aol.com
Sent: September-23-12 6:06 PM
To: vwdiesel at vwfans.com
Subject: Re: [Vwdiesel] brake servo


  Initially it sounds like the servo's bad but then the assist WILL go away
with a few close-together applications of the brake pedal.  More often it's
a weak vacuum pump.  If you have the diaphragm type it could be the
diaphragm starting to go or a leaking valve.  If it's the sliding vane type,
cleaning it could help, otherwise it takes some machinin/filing to get the
clearance to the lid reduced.  The vac line to the servo can get cracks and
leak.  The slow leak (depending on how slow) could be typical, a small leak
around the master cylinder or a genuine leak in the booster.  They seem to
dump a certain amount of vacuum when applied.  Haven't tested to see if
that's a function or from impending demise.
    Loren



-----Original Message-----
From: Dianne Roa <dianneroa at gmail.com>
To: vwdiesel <vwdiesel at vwfans.com>
Sent: Sun, Sep 23, 2012 10:22 am
Subject: [Vwdiesel] brake servo


I have a 1984 VW Rabbit *diesel*.  When I push the brake pedal down, there s
a hissing sound.  The first time I push the pedal down, the brakes work ine.
If I pump the brakes several times, the vacuum assist from the brake ervo
goes away.  I put a vacuum pump on the brake servo input.  I created  vacuum
on the brake servo and it gradually leaks down.  Also when I put he vacuum
on the servo I can hear a slight leaking sound.  If I put the acuum on the
servo as soon as I push the pedal down it goes to zero.  I am ot absolutely
sure if the problem is with the servo or the vacuum pump.
Bob Roa
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