front disc brakes keep locking up!!!

[traie88]

Ok, six months ago, I had the master cylinder replaced and the front disc pads also replaced. Two weeks ago, I was driving down the road and I noticed it was getting harder and harder to go. So, turns out, that the front brakes on both sides were locked up.
I released them by pulling out the two main lines running into the master cylinder and quickly replaced them.

I had the plunger rod adjusted from the linkage to the master cylinder by a mechanic. It was fine for about two weeks, then they locked up again. During this time I noticed a lot of differentiation in pedal pressure...sometimes touchy, sometimes normal. Now the pads squeak really bad. By pulling off the main lines from the master cylinder releases the pads, albeit temporarily, but what could it be? The power brake booster? Is the pedal switch nut out of adjustment? Could the calipers be all out of whack? I don't know.... Can anyone help???