Inconsistent brake pedal travel - Mazda5

I had a pretty strange braking issue on the way to work yesterday. At least it's never happened to me, nor have I heard of it before.

Brake pedal was normal for the first mile of my drive to work. Driving up a steep hill I had to brake unexpectedly when the brake pedal developed 3-4 times it's normal pedal travel - almost to the floor. It stayed that way for nearly the rest of the drive, which is about 30 minutes. Once at the end of the travel the car stopped fine, it was mostly disconcerting. At a stoplight, I was testing for a leak by pressing on the pedal firmly to see if it would continue to drop. Which it didn't, at least not appreciably. After that the pedal pressure was normal again for the last few minutes of the drive. The drive home was completely normal again.

No brake fluid is missing from the reservoir and the master cylinder isn't visible without removing some stuff. I haven't had the chance to have the car up to check for leaks yet, but that seems unlikely at this point.

Any ideas?

Sounds like a booster problem. IIRC, they have a check valve on the vacuum hose from the engine to the booster, maybe that got weak and let out all the power assist at that one stop?

I have nothing to offer for this particular problem, but my Mazda5 always exhibits very long pedal travel when doing panic stops from WOT. Something about the booster and low vacuum, not sure, but I've never driven another car that does it. Unlike your issue, though, it always goes away if I lift the pedal and re-apply.

Hmmm, I was in fact at WOT. Just drove it again and no problems.

Wouldn't an absence of vacuum for boost manifest as high effort, and not long travel?

Following that thought, that means something's moving more than usual; pad knockback would probably go away after the first long-travel application unless it was something weird like a piston seal twisting and pulling the pads back when you take your foot off the pedal.

Do 5s have drum or disc rears?

MC bad.

My parents had a 5, and from new the pedal would travel to the floor if you held your foot on it firmly. let go of the pedal and everything would be fine. The car never had any leaks or other braking issues, it stopped remarkably well for a small van actually. This behavior was only noticeable when deliberately pushing down firmly on the pedal past the point of maximum braking force (such as when stopped at a light). I figured it was some damping feature in the pedal itself or a quirk of the booster design since it didn't affect the use of the brakes at all.

Discs on the back. For what it is worth, I don't have a sinking pedal at stops.

I hate, like always, that it happens sometimes. Always results in trying to diagnose something that won't cooperate with scientific method

Dr. Hess, any thoughts on exactly how with the sporadic nature? I'm guessing that it would be a seal problem on either the front or rear MC piston, which for some reason causes that piston to do essentially nothing, leaving braking to the other piston and using up travel?

I have to admit, that sounds a bit better than a caliper piston seal retracting the piston far enough to cause that some of the time...

Look in the reservoir, black gunk in the bottom?

No black gunk

check you brake pedal.. just get under there and look at it... might be broken.

I've had them intermittently do that when they were in death spirals. As to exactly why, dunno, don't care. Probably seal starting to go bad or going over a rough spot in the cylinder and sometimes sealing. Last one I had go bad was on the Rolla. Bought a "new" MC at O'Reilley's. I was a bit concerned because the box had greasy hand prints on it, but I looked and couldn't see any fluid in it, so I tried it. It was worse than the one I replaced. Brought it back for another and that one was fine, fixing the problem.

I haven't seen this issue on our 5 but I did realize how easy it is to smoke whatever pads I have installed. I want to say they are ceramics of some sort. I was trying to burn off the pad deposition that's felt like warped rotors and after the third ABS threshold stop from 60mph in a row there was a definite reduction in braking to go with the smell.

Well, braking has been normal so far. I was able to put it on a lift at a friend's shop to get a better look at everything and no sign of leaks yet. I guess I'll wait to see if it happens again. If so I might just replace the MC on general principle.

In reply to Matt B:

I'd be sorely tempted to replace it on more than general principle...

It's not that I think you're completely nuts to be a little wait-and-see; I just think:

• the list of things which could change travel arbitrarily is short
• assuming it's the MC, you no longer have a spare circuit when it's doing that
• the circuit that hasn't failed yet is living in the same fluid
• the stakes are high

I like to stress test brakes by practicing panic sops, while parked, at idle(max booster power). If something pops or acts up then, it did it on the driveway, not on the highway behind a semi that cut you off.

I had something very similar happen on one of my vehicles. What it turned out to be was the pad separated from the backing and must have dropped on to the road. The pad fell off and lots of pumping closed up the gap left by the missing pad. A quick inspection did not find it. I did not find leaks or anything unusual either. It became apparent when the brakes started grinding a few days later. Take the calipers off to inspect.

Back from the dead!

Well, this problem didn't happen again until a couple weeks ago (again while accelerating, but not WOT). It had the same behavior except this time it didn't last - just one stop brought it back to normal.

Decided to install a new MC last week just in case. Happened again this morning.

What's interesting is that it isn't a "spongy" pedal or loss of actual stopping power. It's like the activation point is simply (much) lower in the pedal travel temporarily. This morning's event was a little different, in that I wasn't on the throttle and it felt like the ABS kicked in momentarily (vibration in the pedal). I was near a construction zone and there was probably a little debris on the road, but I was only going 5-10mph as I approached a stop sign.

I am due for rear pads, per Shuttlepilot's suggestion I'll take a good look back there when they're changed.

Matt B Did you ever resolve this? I have a 2009 Mazda5 and this has been happening sporadically for over a year. MC replaced. Booster replaced. Four times to the dealer. Of course they can't reproduce the problem. It happened once after ABS in wet weather; once after emergency stop; but many other times just normal city driving coming to a stop sign. Following these incidents, there will then be a "wheezing" sound when I let my foot off the brake usually til the end of my trip and then it seems to "fix" itself, though at times it has still happened the next time I drive.