Rt Frt Brake Drags, R&R'd Caliper, Same!

Hopped off the highway 3 days ago, hot weather, noticed brakes dragging pretty badly. Right front caliper seemed to be not releasing. Purchased rebuilt caliper, installed new pads, bled caliper. Tested fine. Drove to work (20 mile highway drive) & back Monday, all was fine. Yesterday, (Tuesday), was fine in morning, driving home from work I exited highway and same problem returned. Could it be the Proportioning Valve, (hopefully not, on an ABS system), or the Master Cylinder? (I'd rather replace the Master Cyl than the Prop. Valve). It seems like the fluid is not being allowed to escape from the caliper when the pedal is released. I got home, allowed it to cool, and it seems fine now, but will the problem return today? Any ideas? Thanks. -Jim.
Oh, vehicle is 95 G20 3/4T Conversion Van, 99K Miles.

Have you tried replacing the lines?

I have seen the flexible lines that go to the caliper collapse with age and not let the brake fluid return as it should, You might want to check that...

RBRACZYK & RPRESTON, thanks for thte replies. I assumed the lines would be ok since the caliper bled ok, and the brake is being applied with pedal pressure. So could the soft flexible line actually pass fluid one way, but not the other? (Ahh, I guess would make sense, since it would work on the brake apply, because the fluid is under pressure, but on release of pedal, it would be under "vacuum"? Sort of?).
I would have replaced teh line when changing the caliper, but it had no signs of cracking or deterioration on the outside...
Thanks again, I will try it and report back. That will be much easier than changing the Master Cyl.
(BTW, when I drove to work today, I carefully avoided any hard braking at all, coasting as much as possible when I knew stops were coming up. No dragging. It seems like the highway exit on the way home, in heat of the day, causes the problem to act up. Probably because it builds more heat in the caliper & fluid, especially after a 20 mile highway run?)
Best regards, -Jim.