Brake question

Help!!!

I have a 97 GT and the brake pedal was starting to go low. My guess was it needed a new master cylinder. I brought it to the shop and they agreed it needed a new m/c.

My car does not have abs and has a hydrolic booster so I guess the master is hard to find. The shop ordered it and a week later the master came in and they installed it.

Now comes my problem. 2 days after they changed the master the pedal got really hard and the brakes started to lock. I got the car home and the station came to my house and towed it. They looked at it and made a couple of adjustments and said it was fine. The next day driving home from work the same thing happened but this time the brakes really locked up and smoke was pouring out of my front tires. I called a tow truck to pick up the car and when the car was towed to my house the brakes were fine again. It seems like after they heat up the master builds up pressure and they lock.

The shop ordered a new master and it has been a week and they still don'y have it wtf!

My question is does this sound like a defective master cylinder?

Any help you can give me would be awesome!

Thanks

I replaced the front calipers, rotors and pads few months ago. Back brakes are in good shape. Pedal was going almost to the floor at times.

The shop checked all the brakes and said everything was fine. It didn't look like any fluid was leaking. I've never changed a master cylinder before so when they said that's what it was i just told them to do it.