It could well be the drums all around need to be checked for adjustment. If they were loose to begin with and you jammed the brakes on hard, they may have bound up. The other thing may be the wheel cylinders themselves. Over time there is an amount of rust and corrosion/pitting that occurs in the old cast iron wheel cylinders and even if the brakes were bled, the junk in the lines can shift and cause a binding. The other thing to look at is the soft brake lines themselves. Over time one of the inner lines can develop a crack so that when you hit the brakes, fluid builds up between the two linings and when you let off the brakes the pressure will work like a one way valve and block a cylinder from backing off properly. There are three soft lines, one on each front wheel and one to the rear axle to body for suspension travel.