if you went from 4 drums to 4 discs, was there a proportioning valve for the rear line? I've seen (translated as I've done) calipers mixed side to side- it put the bleeder in a position that made it impossible to get all the air out of the piston. Double check that your bleeders are at the highest point on the caliper, you've gotten ALL the air out of the lines, your hoses aren't twisted, no leaks, and that you used the correct fluid. First few thoughts to mind anyway..
Buckle up- I wanna try something!!