Last week my brake pedal started going to the floor when I stopped, it still stopped and felt fine on the top half of the pedal travel but then it would just slowly fade to the floor.

I took it to a local shop and they said the master cylinder was leaking internally and my rear brakes were shot. I have 75,000 miles and the rear brakes were original so I told them to fix it. This is basically what I expected it to be.

They kept thinking they were getting defective master cylinders but after the two Napa parts and two Carquest parts they realized they misdiagnosed it and said I needed to take it to the dealership. I paid for the rear brake job since it needed done anyways and have an appointment for next week at the dealership. Does anybody have any ideas of what might be causing the problem? I see no brake fluid leaks, no vacuum leaks, the booster was replaced 25,000 miles ago due to noise, no ABS light on, no CEL on, no other symptoms.

Thanks in advance! and Happy Independence Day!