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suznal 09-10-2012 10:31 AM

Brake pedal goes to the floor – sometimes
The brakes on my ’05 ZXW (with ABS) work just fine until I’m stuck in traffic.
When I’m “crawling” to get into an event, or when there is highway construction (“UGHH!”) my foot rarely leaves the brake pedal. Instead I release pressure just enough to let the car creep until I have to come to a complete stop again. Sometimes when doing this for a while – out of the blue the pedal will go to the floor! I have to completely release the pedal then re-apply pressure and the brakes work fine again.
This is the only circumstance that the brakes fail, like I said, at all other times the brakes are just fine.

What are some things I should look into?

DerbyDave 09-10-2012 11:24 AM

It sounds like you have an internal leak in your brake master cylinder. With steady pressure the brake pedal should not move, if it does then it is time for a master cylinder replacement.

sailor 09-10-2012 12:07 PM

^^ yep, when you take it apart you may find some brake fluid inside the "dust boot" - that would be extra confirmation of the problem.

"pumping" the brake pedal just to stop ain't fun!

A bit of a PITA to replace it & bleed the whole system, but WELL worth it!

suznal 09-10-2012 12:19 PM

The brake pedal doesn't move under steady pressure, I can hold it for over a minute without movement.
When I experience the problem, pumping does nothing - I have to completely release the pedal and then she's good as new.
That's what has me confused and wondering if (like all these newfangled elctronic gizmos) it had something to do with a sensor or the brain?

sailor 09-10-2012 01:08 PM

as usual, it's hard to tell exactly what's going on from a distance....

Releasing & reapplying pressure to the brake pedal is usually considered "pumping", even if only the one stroke is required - and I think you mentioned it going to the floor.....

Was your steady pressure test done with the car running, or with it shut off??

(Brake vacuum booster &/or ABS can change your feel & how the system reacts....)

For example, you can have a QUITE firm pedal with the car off - but once it's running & the booster is functioning, the pedal might not feel so good....

ABS issues COULD come into play, that's why I mentioned the leakage found at the brake cyl. as a confirmation that you actually have an issue there before pulling it apart completely & replacing the master cyl..


cbdallas 09-10-2012 01:16 PM

The master cylinder is bypassing, but not consistently. Get it replaced before it gets worse.

sailor 09-10-2012 01:33 PM

Considering the consequences of driving it with brakes you can't be sure of, diving in and solving the issue is imperative!

And the cost of a replacement master cyl. & the time needed to install it is a small price to pay when you are no longer confident of it's functioning properly!

Some things you just DON'T want to put off until the repair becomes TOO obvious!

suznal 09-10-2012 01:48 PM

Steady test both car running and off.
I did bleed the lines thinking there might be air - no air found in the lines.

Guess I'll get the Hanes manual out tonite and get ready to start calling around for price and availability.
Thanks for the help!

focusgg 09-10-2012 09:37 PM

I have the exact same problem Suznal is describing, and it occurs under the exact same circumstances.

Suznal - I would be interested in hearing if replacing the master cylinder solves the problem and if you found an other evidence while replacing it that would suggest it was definitely the master cylinder, e.g. visible leaks.

sailor 09-11-2012 12:27 PM

Agreed focusgg, brake fluid above the piston would prove the issue once the master is pulled away from the booster for inspection.

interesting that he can't duplicate the issue in the driveway.....

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