I had the same thing happen to me. When I first went through my car to make it streetable, I replaced the master cylinder with a reman NAPA unit. Not even six months later, it seized up and would lock my brakes if I touched them. I replaced it again and haven't had an issue with it. I would say take the master cylinder back and exchange it for a new one.
Yeah, the return springs are inside the master cylinder. They sit in the end of the cylinder bore and push back against the end of the piston.
