Brake pedal moosh

I have a 1996 Honda Accord with about 180,000 miles–runs great. Recently, after coming safely to a stop at a stop sign or traffic light, particularly on a downhill grade, the brake pedal then mooshes slowly to the floor.

I guessed the master cylinder was starting to go, and my mechanic who knows this car, agreed. So we replaced the master cylinder. The braking improved, and the problem doesn’t occur as frequently now, but at intermittent intervals, it still just mooshes into the floor while sitting still after coming to a stop.

I’m stumped, and so is the mechanic. Any suggestions?

Could be another brake component (caliper, wheel cylinder, etc) leaking. Was the whole brake system checked out before master cylinder was replaced?

Air still in the system. Ask mechanic if he: A. Bench bled the master cylinder. B. Bled it after it was installed. C. bled all 4 wheels. (he could have pushed an air bubble out to one of the calipers/wheel

cylinders.) D. You could have a bad new or rebuilt master cylinder. E. Way outside chance but you never know: If you have some kind of vibration especially from in the front end; then while cruising for a while the vibration could cause a caliper piston to ?walk? back into it?s bore (cylinder) a little. That would

temporarily cause excessive pedal travel. F. Another outside shot: Since you just spent $ on your brake system, could your objectivity be different now as to the way you see your brake system working? In other words, are you absolutely sure you have a problem- no offense.

If there is no fluid leaking from the system and the car does not have ABS it would certainly indicate that the new master cylinder is failing. But I would like to know more of caliper pistons retracting into their bores. I have never seen that but wish to not be too surprised if I do.

I agree with Rod. New parts can be bad out the box too. Defects happen. Where did the master come from? Made in China crap?

Also have they confirmed all 4 wheels lock and release immediatlely when on the lift? Are all the calipers free?

Do the brakes hold well even when the pedal sinks? That would indicate a leaking power booster.

If your fluid level isn’t dropping at all, and 4 wheels brakes are applying and retracting freely, most likely you have a defective master cylinder or your brake booster is leaking.

Once on a Cartalk broadcast they mentioned where a front caliper piston walked back into the bore because of vibration caused by front wheel imbalance on a long trip. I thought this was a little dubious but once after driving on an interstate for about an hour without applying brakes the pedal went to the

floor as I slowed on the exit ramp. Immediately it pumped back up to a normal pedal. Never happened before or since. I had bent steel wheels that produced vibration that I lived with no matter how well I balanced them. I couldn?t find anything whatsoever wrong with the brake system at this point. Vibration

can cause really weird things. I was using a 3/8 drive air ratchet with a long extension and a craftsman swivel socket once. The 3 pieces of the socket were held together with roll pins which are really in tight with an interference fit. While using the air ratchet I watched one of these roll pins literally walk halfway

out of its hole in about 10 seconds. I had never seen anything like that but there it was. I think it has to be just the right sympathetic vibration or whatever term you use for the phenomenon to happen which is why you hardly ever see it.

Thanks. It does have ABS brake system. The mechanics bled the air out of the master cylinder and lines, so I don’t think that’s it. I don’t know enough about brakes to know whether the problem of the caliper piston walking back into its bore (suggested by several responders) could be the problem,but I can certainly check that out. I hadn’t noticed whether it does it after having driven some distance without braking, or whether it does it in instances where I’ve been intermittently braking, as in city traffic. I’ll test for that. It’s definitely better than it was prior to the master cylinder replacement, and not as frequent, but still there occasionally. The sympathetic vibration angle does intrigue me. My alloy wheels on the front end are pretty worn, and it could be that some vibration is causing the caliper piston to “walk.”

Didn’t ask about all four wheels locking when on the lift, but I will. Yes, the brakes appear to hold even when the pedal sinks, but when you’re on a downhill slope, it gives you a weird feeling when it does and I quickly pump to get the pedal back up. I’ll have them check the power booster for leaks. Hope that’s not as expensive as a new master cylinder.

You could have gotten a bad replacement part. Exact same thing happened to me once…