Car Wont Stop After brake pedal is on the floor for a second...CERTIFIED

[dee]

MECHANICS PLEASE!!!? I Have a mazda Protege 94 and i just fixed my front brakes like a week ago. My rear brakes are low at this moment. but this never happened before, all of a sudden i felt like my car wouldn't stop and even if i pressed the brakes all the way down the car will still move until it came to a complete stop, the brake light is on. My mechanic is stupid he's telling me its the back brakes and im wondering if he is certified because i want to get him in trouble for doing a bad job on my brakes, can i sue him for that? well first i wanna know if im right because when he did the job everything was fine i didn't feel no signs of the rear brakes being bad. i had pressure on them and everything, he put brake fluid in and the light had went off, now the light is back on and there's enough brake fluid. what can be the problem? he had also put an used caliper on the right side because mines was rusted and screws wouldn't come out... its really dangerous i was very close to getting into an accident with a truck. please HELP