Date:         Tue, 14 May 1996 11:56:34 +0100
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         verbeek@the-hague.stbv.slb.com (Brian Verbeek)
Subject:      pressure-relief doohickey

So Steve Maher said:

>Do newer beetles have such a pressure-relief doohickey?
>In what year did the pressure-relief start, for buses and/or bugs?
>Where- in the master cylinder?

It is sometimes referred to as bottom valve. Look at cut-away drawings of a
drum brake master cylinder if you're really interested. I'm thinking back to
my school years now, I think that the drum brake systems had the valve and the
disk brake systems did not. This is what happens: When braking you build up
pressure, after releasing the pedal the brake fluid starts to flow back into
the master cylinder and the reservoir, just before the pressure drops to zero
the bottom valve closes as to keep some rest-pressure in the system, the
objective is to keep system pressure slightly higher than atmospheric, so when
there is a 'leak' (very small, more like 'sweating') it would be brakefluid
leaking out of the system instead of moisture, dirt and/or air creeping in.
This is neat of course, drum brakes have springs that keep the shoes against
their adjusters and wheel cylinders, the rest-pressure is not enough to overcome
this spring pressure, the shoes cannot touch the drum. On a disk brake, however,
the springs function is more or less taken by a rubber ring around the piston
in the caliper. This ring deforms a little bit while braking and when the
brake pedal is released the ring wants to come back to its original shape
and pulls the piston a little back so the pads clear the disk.

Ring in Rest:                When braking:

   ---------| |------             --------| |-------   = Caliper Bore
            | |                           \ \          = Rubber Ring
   ---------| |------             ---------| |------   = Piston Wall
   ------------------             ------------------

The rubber ring is not able to pull the piston back if there is rest-pressure
behind it, the pads would touch the disk when the system is in rest.

So when converting from drums to disks: remove the bottom valve (of the
circuit that actuates the disk brakes) or swap the master cylinder
completely.


Brian


PS, I have a spare '71/'72 SuperBug disk brake setup, original VW.