I had a single sticking piston in a 4 caliper willwood front disk system recently that created a hard pedal and lack of clamp pressure ... I decided to rebuild the calipers with willwood seal kit and reassembly lube. The hard pedal problem disappeared as did the static brake drag I was getting.
If you havn't allready solved your problem, jack up the car and see if you have a wheel that wont spin when the brakes are not being applied.
If you find one, you have found a sticking piston or two or more....rebuild ( cheap and easy if the caliper has rebuild kit ... mine was 16$ for two calipers and assembly lube from willwood) or replace the calipers altogether.
Experts would insist you do both sides to keep things even...no matter which route you go...so I did...figuring if one side was sticking, the other was soon to follow so...why not.
Bo-peep 68 camaro 396BB.