does it still do this with the new master cylinder?

ive heard of this problem and its not really related to abs, that could be another problem. what ive heard is that the bypass port/channel inside the master cylinder gets clogged or covered and as the fluid heats up (from ambient air and under hood temps) and expands it can not go up into the resivouir and there fore applies the brakes with out input from the driver.

ive heard of it once on a mid 90s 2500 silverado. it would happen after about 30-40 minutes of driving. the brake application wasnt gradual, it was all at once and it was full brake. not sure if its the same thing you have going on, but i figured it may be something to look into.