I just bought my 2003 gand am GT in july.brakes started sqealing and grinding after 3700 miles.they put the wrong ones on.I changed them and now the pedal goes to the floor when the car is started.nice and tight when car is off. I bleed the brakes ,air is out. caliper pistons seem to be working.everything seens fine till the car is turned on.I am at a loss please any suggestions would be greatly appreciated.