91 z34 brakes

This is my son's car which is at the mechanic's right now. The mechanic is having trouble figuring out what's wrong with the brakes. Only when they're cold - it takes a LOT of pedal pressuer to make the brakes work. It seems to be all 4, not just front or rear. He has checked and thinks that the calipers, the rotor finish, and the vacuum supply brake booster are all OK. He's not sure that it's the master brake cylinder ($200 part - don't want to spend if not the problem).
Neither my son or I are mechanics and we're going to sell the car as soon as this is fixed.
Does anybody know what causes this?

I would bet the rear calipers are siezed and not adjusting. When the brakes get hot, everything is expanded, so they work better.