Overzealous stopping

Ok this has happened more than once so I think it is time to sort this out. Yesterday on my weekly drive around the block to make sure everything still works my left front wheel locked briefly as I tapped the brakes backing down the driveway and then once more as I approached my first stop sign. The rest of the 4 mile journey was uneventful, stopping was normal with no brake pull or fade. The details: 1971 Swinger with disk brakes from what I believe was another mid 70's A Body. They are single piston.
The whole setup is manual and I still have drum brakes in the back. On occasion the right front has squealed under light braking but never under heavy. The pads have very little wear on them and other than a visual inspection this is the one part of the car I have not completely disassembled at one time or another. Could this be a caliper piston sticking or a problem with the pins that attach the caliper? Heck I probably should find an exploded
view picture to verify they were even installed correctly. Thanks for the help.