Dragging brakes

[zm@mhcnet.att.com (Zafer Mehmood [209])]

	A few weeks ago I had written about a strange problem with my '87
5000 TQ. Essentially, the brakes begin to drag after about 20 to 30 minutes
of driving on a hot (> 80-85 degrees) day with the A/C on. The car feels
sluggish and eventually, the front rotors get hot enough to begin warping
and the steering wheel begins to vibrate (shimmy). A few minutes stop to
cool down, releases the brakes. Seems like brakes on all wheels are dragging,
with the fronts getting much hotter than the rears. I ruled out the possibility
of one sticking caliper due to this.

The pressure accumulator was bad and I changed it and flushed the hydraulic
fluid. Also bled and changed the brake fluid with Castrol LMA Dot 4. Problem
didn't go away. Seems like there is some pressure build up in the system
when its hot. The dealer checked the proportioning valve and said its fine
and suspected the brake master cylinder. But before I go replacing one part
after another,  I want to solicit your help and suggestions.

I have noticed that after 15 - 20 minutes of driving (engine temperature at
normal warmed up range), the brake master cylinder gets very hot - I can
barely keep my fingers on it for 2 or 3 seconds. The ABS unit, the hydraulic
pump and the hydraulic fluid reservoir also get quite hot. I am wondering if
this is normal. Could others with similar cars please check and tell me if
these components get hot within 15 - 20 minutes of driving a cold engine?

The problem seems to be directly related to under-hood temperatures. The
last few days, the temp has been 70 to 75 and no problems with the brakes
dragging. Even on a hot day, if the A/C is not used the brakes don't drag
or a lot more driving is needed to cause the problem.

Any help will be appreciated.

Zafer

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Zafer Mehmood				   AT&T Bell Laboratories
zm@mhcnet.att.com			   Murray Hill, NJ