Another "brake help" thread...

A few small points. As mentioned, the fill port ("inlet" in post 25 figure) must be exposed when the pedal is released. To insure this, insure that the brake lamp switch is only sensing motion of the pedal and not limiting it, i.e. the pedal should not bottom-out on the switch. That is a common mistake. If the fill port isn't exposed, that causes a "hydraulic lockup" where the fluid is trapped so can build up pressure. The calipers can drag to generate heat, which expands the fluid, to cause even more braking. Most common with a power booster when the output rod tip isn't adjusted properly so the MC piston can't retract enough to expose the fill port. For those with a booster, there is a special tool to measure dimensions to adjust the booster tip ($15 ebay). But, not the problems you describe.

Another point is that, unlike drums, there is no spring to retract caliper pistons. Instead, the square O-ring around the piston is supposed to tilt as the pistons move out and rotate back to retract the pistons when fluid pressure is released. Sounds quirky, but works amazingly well. Less so if the O-ring ages so the rubber gets hard. Problem would be the calipers sticking, which also isn't your issue.