High temperatures affecting brakes

[Bernard Littau]

> >I have a 1989 Audi Turbo Quatro.  Using the airconditioning with high
> >ambient temperatures after 10 to 30 minutes the brakes begin to
> >drag.  Pulling to the side in neutral the car will come slowly to a
> >stop.  After turning the engine off to allow things to cool down the
> >brakes will slowly release.  The brakes have been blead and the fluid
> >changed.  Not sure where to look from here.  Has anyone had this
> >experience.
>
> Your brake master cylinder may be deforming slightly from the high heat in
> the engine compartment.  Carry a quart of cold water with you and pour it
> over the MC when the symptoms reappear and see if that doesn't do the same
> thing as your long wait for everything to cool down.  If that brings
> instant relief, you probably need a new MC or figure out a way to
> ventilate
> the heat from it.

I agree the heat may be deforming something, but the problem is likely that
the plunger depth for the Master Cylinder is not adjusted correctly.  As
things heat up, the plunger may be getting longer, and closing off the
cylinder vent to the brake fluid reservoir, thereby allowing brake pressure
to build even when the pedal is not depressed.  The vent could be closed off
all the time, and the extra heat is now causing the pressure to build to the
point you notice brake drag.

You should take off the MC, and check the plunger depth, and adjust if
necessary.  Sounds like it needs to be a bit shorter.

It's also possible that the vent hole in the cap to the reservoir is
plugged, so check your cap first.  Next time the brakes drag just open the
cap and see if that immediately "cures" the drag.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq