Intermittently Low Brake Pedal

[Devin]

I was having a problem with my brake master cylinder. It would lose pressure at a stoplight and go to the floor. I replaced it and it seemed fine. But then I noticed that sometimes the pedal is really low and soft and sometimes it feels fine. It doesn't have a correlation to run time or heat. It's just random. I didn't think this was air in the system (I did bench bleed the unit) because that would be more of a consistent soft pedal right? Perhaps it's a faulty master cylinder? Or does it sound like a vacuum issue?
Thanks

[dlb]

so you replaced the master cylinder and now you're having a different brake issue? was the new master cylinder new, or a reman? my guess is the new unit is faulty.

when you say the brake pedal is now low and soft sometimes, does it also lack braking power at those times? do you have to stand on the brakes to make them do anything?

[Devin]

Yes this is a different issue than before.
I think it was a reman unit.
Yes, the braking power is reduced during those times.
I'm thinking of just replacing the whole thing with a new one. Does anyone have any part numbers for a good new master cylinder?

[dlb]

xirdneh just bought a new one off rockauto for $27. there might be a few left. the thread is prob in the repairs section. take a look.

[irowiki]

Did you bleed the brakes at each wheel?

My 83 was doing the same (sloppy brakes, pedal randomly going to the floor), bleeding every wheel ended up fixing it.