ok so i have taken my car to 2 mechanic shops now. nobody can tell me whats wrong. my brakes work but they fail sometimes, usually at low rpms and especially while parking. if i hammer them to the floor they still work. most of the time there firm and work well but then all of a sudden they go right to the floor. i just replaced the master cylinder. they've been bled. its non abs so what the hell can it be. ive heard proportioning valves never go? if i did how cud i tell...any help is appreciated

 

Test your brake booster check valve. It's supposed to hold vacuum so you can have brakes at lower RPM's (or if the car shuts off).
Run the car for a few moments and then shut it off. Pump the brake pedal. If the pedal firms up, the valve is good. If not, it's bad and should be replaced.
Same thing happened to me and it took me a good while to figured it out.
Good luck!

 

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