the way the rear disc are made it could be anything
are they not geting any pedel at all? on my 85 with disc the rear caliper exploded are alot of times the rear stick, if you turn the pistion the wrong way they explode, if the pistions went back ez then i doubt thats the problem. did he do a swap? or is it a factory disc break car? if he swaped the perpostioning valve is diffrent. i really need to know whats happening or not happening to give you a better idea what to look in to.
http://home.earthlink.net/~saawatson/ my 1983 trans am. PONTIAC POWERED