MC Master Cylinder heat causing brakes to apply.

[Doyt W. Echelberger]

Here is Bernard Litteau's reply to my earlier call for help on binding
brakes. Thought you might be interested in an explanation about what causes
the binding.   Doyt Echelberger

"For the brakes to bind as you describe, assuming the other stuff is OK, then
>the MC must not be returning to its fully retracted position.  When the MC
>is not being actuated, the internal cylinder is all the way to the back, and
>the slits are open such that the brake lines communicate with the brake
>fluid reservoir.  If the MC cylinder does not retract fully, these slits
>stay closed.  In order to have the brakes bind, you need a combination of
>the slits being closed, and some sort of pressure build up in the now closed
>hydraulic system.  Enough heat will cause pressure in such a closed system
>to make the brakes bind.
>
>Sounds like the Audi MCs are prone to becoming sticky due to heat
>distortion.  Scary.
>
>I would think that one could note excess play in the brake pedal if the MC
>were not fully retracting -- it would take more brake pedal travel before
>one sensed the onset of real resistance.
>
>Best,
>
>Bernard Littau
>Woodinville, WA
>'88 5ktq


 >>>>>>>>>>>>>>>>>>>>
At 10:24 PM 6/27/2003 -0400, you wrote:

>Hey gang:
>Today's episode. '91 200 20V Sedan.
>I read about it this month and now I've felt it first hand on the central
>artery in Boston.
>Temp gauge said 97 and the car inched forward in traffic, but I didn't need
>the brakes to stop.
>The car would lurch to a stop like the brakes were firmly applied.
>The car struggled in 1st gear at idle to move.  The engine "lugged"
>Then it went away only to come back several 100 feet later.
>The ambient temps in the tunnel went to 107. (FYI: coolant temp stayed at
>half mark w/fan running)
>Wow, this is Boston and we had the windows down as the
>glass green house made more heat than the AC could cool.
>
>So I pulled the trigger and abandoned the "stop and go" commute to the Cape
>because of the dragging brakes.
>I parked in our parking garage and let the vehicle cool.
>Now down to 79 degrees.
>With-in twenty minutes I could rock the car in neutral, where earlier it
>wouldn't budge.
>Quite an experience.
>Any ideas on how to combat this?  Home made heat shield?
>Brake flush with higher temp fluid?  Drive in the winter?
>or am I looking to replace the MC?
>TIA -Scott in BOSTON