brakes dragging question

[RustyBucket]

A pretty serious illness left my 2 old trucks pretty much abandoned in the shop for a year+. Recently I charged the batteries, primed the carbs. washed 'em off and amazingly both still run/shift/drive/ pretty good. Aftermarket A/C on the '71 GMC still blows cold. Driving the GMC the other day & stopped and talked to a neighbor for 15min or so. I left the truck running w/the air on and the temp. got up to about half way on the factory gauge. I started to back out of his driveway and the truck didn't want to roll, the front brakes were dragging. I barely was able to drive around to my driveway 'cause they were almost locked up. I left it running, raised the hood and ran cool water all over the radiator for awhile. I brought the temp. down to the first mark on the gauge, closed the hood and the brakes were fine. Yesterday I backed it out & let it sit and warm up to almost halfway on the gauge. The brakes bound up again. Truck has factory pwr. disc brakes. I'm not healthy enough to work on it much yet, but has anybody had similar brake trouble? THANKS in advance!