Did they change front or rear brakes? If it was only one or the other, it could be your proportioning valve is over adjusted to one or the other. Could just be bad rotors too. I had problems with my master cylinder, it failed at about 26000mi and let me with no brakes on the highway
I made it home ok, because I could still get pressure when I slammed on the pedal hard enough and I had my e-brake of course.