Reply to "Brakes locking up"

To add to Larry's deductive reasoning suggestions.

1. Expose the car to heat (leave it in the sun or drive it) then when you know the brakes are engaging, jack it up front and then rear to establish exactly which calipers are binding.

2. Do same with engine running and not running

If it is just front or rear (I assume a pair as you haven't indicated it pulls to one side) the problem is likely not in the MC/booster, conversely if all calipers are dragging it probably is an MC/booster issue. If it only happens with engine running it is more likely a booster issue.

If it is just front then it may be air in the system which is expanding with heat. I seem to recall issues with people not realizing that there are two bleed screws on the calipers and it can seem like all air is out of the system when it is in fact not if you don't bleed both ports and hence there remains an air pocket in the caliper. Someone correct me if I'm talking total jibberish here...I'm on too many forums now

If it is just rear then could something on the e-brake bind when hot? (although normally one would expect the cable to sag when hot)

Another possible if it is either front or rear only is the shuttle valve (different to proportioning valve, still in the trunk, but under the booster). When 'shuttled' (activated when brake pads are too thin) blocks off flow to either front or rear brakes. In doing so I suppose it can seal that side of the system and any internal expansion has nowhere to go other than apply the calipers. The shuttle valve can be activated from over zealous pumping when bleeding the brakes if a hard pump is applied when the bleed screw is open. It is a bit of a bitch to recenter and some people have done it with compressed air, but more often than not requires disassembly of the valve and manual recentering.

Good luck,