Weird Honda Fit brake issue

We just had the master cylinder replaced on SWMBO's '09 Fit Sport, due to the pedal occasionally sinking to the floor while stopped. However after she got it back something weird started happening - occasionally while using the brakes to decelerate, or while stopped, she feels the pedal start to sink down, but then pushes back up. I checked google, but all I found were many discussions stating "they all do that"...we've had the car since new and I've personally put ~100k out of the total 165k miles on - I can tell you with 100% certainty it did not do this previously.

I've not had a chance to drive it, but she took it back to the mechanic who also stated "they all do it"...

Any suggestions on what the problem might be?

The "H" isn't slanted?

Trapped air? If they didn't bench bleed the MC this is a possibility, there also could be air in the ABS if equipped.

My 2013 Fit (same generation) does nothing like that I can feel. The only thing I could possibly think of would be a brake booster-related fault; perhaps a sticking vacuum check valve? Change in brake boost when the idle kicks up?

Thanks everyone(except Bobzilla ). Air in the system was my first thought, but he wrote on the ticket he bled the brakes - and by her description, SWMBO saw him bleeding them with the help of an assistant.

What order did you bleed the brakes?

Is the Fit a cross-split brake system? I'm guessing so.

http://www.mazda3forums.com/110-suspension-brakes-technical-information-faqs-how-tos/394323-how-how-you-really-supposed-bleed-brakes-your-mazda3.html

Try that for grins(follow the instructions I posted for checking if its cross split or not) before spending money elsewhere.

Edit: my Mazda2 is like this system too, and it has rear drums which I thought was odd, but there's some funny business with it too, but both fronts are definitely NOT on the same circuit, so it gets bled like this.

If you've got air in the master cylinder or ABS a simple 4 corner bleed probably won't get it out.

Kenny_McCormic wrote: If you've got air in the master cylinder or ABS a simple 4 corner bleed probably won't get it out.

Reverse bleeding could help.

In reply to Stefan (Not Bruce):

Yeah, that can work, when bench bleeding I prefer to reverse bleed with a big syringe, rather than use the "pump the hell out of it" method. Sometimes you can the air out an ABS box by activating it on gravel for a while and then bleeding normally.

I've had 5 new Hondas since 09 and they've all done that. Maybe you just never realized it and now that youre conscience of the issue you only notice it? Humans are weird in that aspect.

Neither of my fits did this. I remember the OP posting this on fitfreak. (I think that's you) either somebody didn't bleed it right or a faulty part at hand. Which I doubt. Honda makes good quality stuff.

Clearly the Fit is go and it doesn't want to stop.

I've never had a pedal push back up from air in the lines. Squishy yes, but never push back up.

I have had certain drum brakes that could do that. Particularly when the car stopped and rolled back. The lead pad (well, trailing when rolling backwards) would be torqued via the drum into the wheel cylinder (single piston), and pushed the wheel cylinder back in the bore, which would push the brake pedal up a bit under my foot.

I have also had master cylinder boosters that could produce weird feelings underfoot from variations in boost (vacuum). Especially if the one-way vacuum valve wasn't sealing.

In reply to Trackmouse:

It wasn't me on Fitfreak...

petegossett wrote: Thanks everyone(except Bobzilla ). Air in the system was my first thought, but he wrote on the ticket he bled the brakes - and by her description, SWMBO saw him bleeding them with the help of an assistant.

but if he didn't bench bleed the MC prior to installation then odds are he didn't get all the air … while it's not impossible to get the air out of a system after a MC replacement … it's a REAL PITA … and a major time consumer

Pressure bleeders FTW! As long as you attach the correct adapter for your fluid reservoir and make sure its sealed before you begin, its virtually impossible to accidentally inroduce new air into the circuit during the bleed. They hold about quart of fluid and are the easiest way I have ever found to ensure that ALL the old fluid and all the air is out. And you don't have to bother SWMBO to pump the pedal for you

Our 2007 Fit does that every once in a while. Freaks me out every time.

WildScotsRacing wrote: Pressure bleeders FTW! As long as you attach the correct adapter for your fluid reservoir and make sure its sealed before you begin, its virtually impossible to accidentally inroduce new air into the circuit during the bleed. They hold about quart of fluid and are the easiest way I have ever found to ensure that ALL the old fluid and all the air is out. And you don't have to bother SWMBO to pump the pedal for you

of course that also depends on whether or not there is an adapter for your MC … never found one for the CRX … I think my bleeder is in box in the back corner of the shop … covered up with who knows what