MG Car Repair: clutch engagement, clutch release, dot 5

Question
I have a 78 MGB, non overdrive. The previous owner changed the clutch before I purchased it. The clutch engages about a third of the way out in normal driving around the city which I consider normal. I can drive for hours like this, however if I take it on the highway for an extended run for a few hours, the clutch starts to engage about two thirds of the way from the floor and gets progressively worse. If I stop the car for a few hours and let things cool down, the clutch behaves normally again. Why does it change the engagement point when on the highway the clutch is hardly used.

Answer
Hi Allen,

I doubt the clutch itself has anything to do with that. Most likely something is changing with the clutch release system which is hydraulic. You are not using DOT 5 fluid are you?

As far as the hydraulic system it is considered the better system that engages further from the floor and engaging closer to the floor is a sign of problems but not a third of the distance from the floor. If it gets closer to the floor you have a problem.

I replaced several clutches in MGB's a week over a period of ten years and never seen that problem before. The only related problem I seen were from people who used DOT 5 fluid and I suspect that the DOT 5 fluid was affected by heat. But that is just a thought.

Howard