Brakes Dragging When Hot

[theastronaut]

Ever since I got my '66 C10 back on the road I've noticed that the brakes drag once they get some heat in them.

Everything except the hard lines are new. The hard lines were in good condition, no rust. I filled them with brake cleaner and blew them out with compressed air multiple times until the cleaner came out clear. The lines are free flowing with no restrictions.

It has a CPP front disc kit, stock rear drums, and a stock replacement single circuit master cylinder, no booster.

I have verified that there is plenty of pushrod free play multiple times. I even added 1/8" shims between the M/C and firewall and drove around and the brakes still held pressure once they warmed up. I can unbolt the M/C completely and it still holds pressure until it's cooled off.

At this point I'm thinking the master cylinder is machined wrong internally, like the piston can't fully return? Has anyone else experienced this? I've searched and found cases where either the booster pushrod or the pedal pushrod was too long, but I've ruled that out by unbolting the master cylinder completely so that it's not contacting the pushrod at all. If it were the caliper slides binding I'd expect one side to be worse than the other, this seems like it's holding pressure in the lines- both wheels are equally hard to spin if I jack the truck up after driving it, and I would think the slides would bind when cold or hot?

The problem has limited me to only driving the truck short distances so I'd like to get the issue solved soon.