If its just locking one wheel it's the wheel cylinder or shoes in that drum. If it's all the wheels, as Tim says it's almost certainly the master cylinder. I had this on a Jensen once - 150 miles from home on a Sunday afternoon, the front brakes would release after about 10 minutes. I ended up clamping the hoses with 2 sets of mole grips and driving home on the rear brakes.
:oops: :oops: