I believe the piston can only move back to the stop/lock, correct? If the rod was pushing on the piston it would hold pressure cold or hot. I agree that fluid expands with heat. Just trying to wrap my head around it. Strangely enough my new to me RT was doing the same thing, I had to introduce some air into the system to be able to drive it home. I'll fix it when I get it here. I cant say that any of my other old cars experience anything like the pedal changing when they get warm. I built a Vette years ago that I had the issue with and it was an issue with the lines being too close, not to mention I have seen it a number of times professionally.