[Vwdiesel] Weird Brakes / BTDT

MWoodland at Dentsply.com MWoodland at Dentsply.com
Thu Jan 3 07:18:39 PST 2008

Weird Brakes
Been there, done that.
You didn't mention the vintage of the rabbit, but on my '91 Jetta, it ended
up being the (adjustable) actuator rod inside the booster.
(other years / models can adjust inside the cabin.. I've heard)
This should be adjusted for approximately 2 mm (.080") free play between
the tip of the rod, and the socket it fits into in the Master Cylinder.

You can't feel the free play by hand, due to the internal return spring in
the booster. You have to remove the m.c., and make a series of
1. from the face of the booster to the tip of the actuator rod
2.  a. from the mounting surface of the master cylinder to the rear of the
m.c., and
      b. from the rear of the m.c., down to the seat of the m.c. piston cup
/ rod seat

a-b will give you the flange to rod seat length.
Measurement #1 will give you the flange to rod length, which should be (at
least) 2mm longer.
In my case, it was an interference fit by ~ 1mm, which did not let the
piston return enough to uncover the hole to let the fluid back into the
After 15 minutes of driving (and braking), the fluid would heat up, expand,
and apply the brakes.

On my '91, I adjusted the rod by having someone push the brake pedal to the
floor, with the M.C. removed.
This pushes the rod out far enough to grab with  needle nose vice grips,
and the tip adjusted with a 10mm (I think) socket.

My '91 was doing it when I bought it (new Bosch MC), and I didn't get it
right until I had replaced both front hoses, R.F. caliper, booster (used),
and the M.C. (with an inferior generic M.C.)

The issue with hoses is that the O.E. German brakes hoses react with the
brake fluid and, over time, swell internally.
M.C. pressure will overcome the restriction and apply the brakes, but not
allow the pressure to bleed off, back into the M.C.
BTDT also, though it wasn't the issue in this case.


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