It sure sounds like the problem lies with the brake master cylinder. Most likely its not allowing the pressure to release. I recommend replacing it, then make sure to adjust the rear brakes and bleed the system.
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Thats because there are two chambers in the master, through the way of valving.
There is a primary and secondary valve action, the primary is for the rear (they should apply first to avoid "nose dipping) and the secondary for the front.
First off,,,are you sure the rear brakes are fully adjusted up? If not the fronts are doing all the work, and as they get hot, the fluid expands and can be applying the front brakes. Let me know...
Go ahead and check them, adjust them up untill they have a slight drag and get back to me then...lets start there before we go discussing other possibilities :-)
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