binding hydraulic clutch--only after long, long drive

[Joel Skousen]

 I had many Audi 5ktq vehicles and never ran into this problem before. 
When I drive my 88 5ktq Avant over 8-10 hours, any weather, the clutch pedal 
stops coming all the way back up at some point.  If I let it sit for an hour 
or two, it finally comes back up on its own.   Last year I replaced the 
master clutch cylinder, didn't help, then I replaced the slave.  It is still 
happening.  The clutch acts like normal at all other times.  Could this  be 
a heat problem? or does anyone else have an idea why this should be 
happening, even with new cylinders?   Any help would be appreciated.

Joel Skousen