Look for a bent caliper mounting bracket,uneven caliper to rotor alignment or possible loose rotor that has some movement.Sometimes one piston thats not holding tight pressure will keep air in the system.We had a Hemi Roadrunner that we couldn't get the brakes bled on and it turned out to be his wheel studs didn't allow the lugnuts to clamp the wheel and rotor tight.Another car had a bent caliper mount and did the same thing.It's worth checking.