sticking brakes

after driving a short distance my brake pedal gets really hard and when i use the brakes they will stick.i thought the caliper was stuck so i removed it and tried to push it back in using a c clamp,but i couldn't budge it.so i removed the brake line from caliper and the caliper pushed back in easy.i replaced the master cylinder and the problem was solved.
but now after a month i'm having the same problem again,the pedal gets real hard and the brakes start sticking,could it be that the master cylinder is gone again(it was used) or is there some other problem?

it's probably an air lock in the system. Try bleeding the brakes but make sure you start from the farthest corner from the master cylinder and work your way towards it. I think the MC is on the left side so start on the right rear wheel, then left rear, right front, left front. Also when adding fluid to the master cylinder make sure it is a smooth flow so you don't get air bubbles in the system. My guess is when you replaced the master cylinder you got rid of the air lock and thus the problem was solved. It could also be your brake bias valve causeing problems so if bleeding doesn't work try that and check the master cylinder. Good luck

i've already bought and replaced the master cylinder everything is fine,but i wish i had known this sooner i would have tried it. oh well it was only $30.