Now brakes are locking up

[greygoose]

After all the problems I had with the carb and after getting that problem solved.the brakes have started locking up,so bled the brakes and on one of the calipers no fluid would come out so i changed that one.It started work find for a short drive then it started again on the same side or one side is harder to turn than the other.So my question is what next.Should I replace just the booster or master cylinder as well or could the be something else that's causing the problem. Thanks