Why do my brakes lock up?

i Have a 91 accord dx front disk and rear drum brakes no abs. the first time they locked i parked fine and when i got out of the store the brakes where lock. then like an hour latter they unlock by them selfs maybe they cooled down or something. then the next day i drove it and it was fine untill like 10 minutes after the locked up litle by little untill they where stuck all the way. then when i got home i bleed the brakes and push the caliper thing in then now they work the pedal isnt hard but i havent driven it what can be wrong?