1961 Plymouth Belvedere Rear Brakes Tighten on a 1959 Plymo

I recently converted my 1959 Plymouth Belvedere to disc brakes front and rear. For the first couple months things were fine. Recently, the rear brakes tighten, and this seems to be associated with the engine warming up (the car rolls free when first started, but not after the engine is warmed up. I can pump the brake pedal a lot before the engine warms, and this doesn't seem to result in tight rear brakes; conversely, I can let the car warm-up in the driveway, then the brakes tighten aftern a few pumps).

What could cause the rear brakes to tighted like this?

Thanks,

Jim