Classic/Antique Car Repair: BRAKE DRAG ON 1976 FIREBIRD, drum brakes, residual check

Question
WHAT WOULD MAKE MY FRONT BRAKES DRAG BUT STILL WORK AND NOT LOCK UP?  WE'VE CHANGED THE CALIPERS, BEARINGS ARE FINE, ETC.  WOULD PROPORTIONING VALVE OR A PROBLEM WITH THE MASTER CYLINDER HAVE ANYTHING TO DO WITH THIS?

Answer
First of all how are you determine that the brakes are dragging? Disc brakes do not retract as completely as drum brakes until you roll the car under power. My test is with the car on a very minimal grade, release the brakes. If the car rolls it is OK. But when you jack up the car the wheels will be difficult to turn by hand. If they are dragging, you need to determine if the cause is mechanical or hydraulic. To check loosen a bleeder screw. If you get a squirt of fluid then there is a pressure build up. Fluid should drip, that is ok. If there is no squirt of fluid the cause is mechanical. If there is a squirt of fluid there is a problem. A squirt of fluid is acceptable on drum brakes but not discs. Drum brakes use a residual check valve in the outlet port of the master cylinder to keep several pounds per square inch on the fluid to keep the lips of the wheel cylinder cups pressed against the wheel cylinder bore. If someone mixed up the hydraulic line or the cylinder was replaced and there is a residual check valve in the disc brake port then we got the problem. Remember that the front brake outlet port on the master cylinder is the one closest to the firewall. I that is not the problem there could be a problem in the proportioning valve or the master cylinder. Make sure that the pedal linkage is free and that the pedal is returning fully as well.