Re: Brake calipers locking
I hope this helps.
These are classic symptoms of a master cyl that is not returning fully. Some boosters have a pushrod that is a bit too long preventing master cyl piston return, which prevents fluid from being vented back to the reservoir. The first thing to check is that nothing is preventing full pedal return, check the brake light switch adjustment and any rubber bump stops. Power brakes do not use a rubber bumper for the pedal, the booster pushrod stops the pedal and controls pedal height. The clevis on the brake pedal has a small amount of adjustment available. Next pull the master cyl slightly off the booster, if the booster pushrod to master cyl is a bit too long you should be able to feel it when pressing the MC into position. If you drive the car and the brakes drag, undo the two nuts and pull the master cyl forward a bit and see if this helps. I had to grind the rod a bit on the last booster install I did. .010" is the proper clearance here.
There are two lengths of MC rear holes a deep one and a shallow one, and two booster pushrod lengths to match them. A cross match of them usually prevents rear brake bleeding and you can't get much pedal at all. So I dont' think your problem is a complete mis-match. Probably a length issue.
Another problem might be a bad brake hose or pinched brake line, it would act like a check valve, but it would do it all the time, not just hot.
Sometimes the caliper can be miss aligned to the rotor and if a brand new outer pad is put on, the caliper cannot move outboard any more to get enough running clearance. Pickup truck pads are slightly thicker than car pads but use the same identical shape, so this easy mixup can cause pad drag.