Please help!! front caliipers locking up and don't know why

[68firebird]

I've got LS1 brakes on the front of my firebird, I've got a new disc/drum master and prop valve, I have finally taken the car for a few rides and the front brakes stay locked, its happened the last 2 times I've taken the car out. I THINK that the calipers release after the car sits. It did it to me on friday night, sat all weekend till today and it did it again, raised the front of the car and can't spin either wheel. the back wheels seem to spin freely. any ideas?