What would cause a brake pedel to fade all the way to the floor under constant braking? The brakes still grab, but the pedal sinks.
What's strange is that this only happens one out of 20 times I brake, the rest of the time the pedal is firm.

your help is appreciated

 

If you are not losing any brake fluid, then it can only be the master cylinder. I wouldnt wait too long to repair this....one time it could be no brakes at all!
It is possible to replace the rubber seals, but usually the M/C will be rusty/scored and it will be best to replace the whole unit.
Dave

 

this happened in my sister's prelude...replaced the master cylinder and it was fine. i noticed that if i pumped the brakes on her car, it would build up pressure, but it would sink to the floor when i would hold it down.

john

 

As has already been said, master cylinder (assuming no other leaks). Best to use a genuine Mazda replacement as the quality of the parts store rebuilds can be marginal.

 

i had the same problem....my master cylinder was leaking....i guess i need to replace that as well, before i get mine back on the road