I have a 96 geo prizm lsi. With the car not running the brakes have good pressure deprssing the pedal. While running the brale pedal goes to the floor. The car does stop but there is no half brake as such. The brake pads and mechanisms look good. The master cylender was replace a while ago before I got the car. I did notice that while at idle when pressing the pedal there wa a change in the engine rpm. Any remidies?