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-   -   Brake pedal sink at stop lights (http://legacygt.com/forums/showthread.php?t=210292)

cww 07-02-2013 01:00 PM

Brake pedal sink at stop lights
 
Occasionally, when I stop at a stop light, my brake pedal sinks slightly. This happens when the vehicle is idling at a complete stop, with my foot on the brake pedal applying light to moderate pressure. It's difficult to gauge how far it moves but it feels like 0.5" to 1" and then it's firm again. I've been unable to replicate this problem deliberately, but it's happened ~5 times in the last month.

I don't see any fluid leaking around the master cylinder / booster area. Is this symptomatic of a failing master cylinder (internal leak)? I found this info using Google that seems to support this theory:

http://forums.rennlist.com/rennforums/928-forum/685023-strange-brake-problem-pedal-gets-soft-at-lights.html

http://www.hondaaccordforum.com/forum/general-tech-help-7/1995-accord-brake-pedal-goes-soft-idle-34533/

http://forums.evolutionm.net/evo-tires-wheels-brakes-suspension-sponsored-tire-rack/511555-weird-problem-brake-pedal-softens-hardens-randomly.html

I also get a steering wheel shake on the highway exit ramp (~70 to 20 mph). I understand that this could be due to several things, but maybe it is relevant.

RallyMachine 07-02-2013 03:02 PM

Have you brought your car to the dealership for the warranty check/fix on the brake lines?

AZP Installs 07-02-2013 08:11 PM

Quote:

Originally Posted by cww (Post 4490134)
Occasionally, when I stop at a stop light, my brake pedal sinks slightly. This happens when the vehicle is idling at a complete stop, with my foot on the brake pedal applying light to moderate pressure. It's difficult to gauge how far it moves but it feels like 0.5" to 1" and then it's firm again. I've been unable to replicate this problem deliberately, but it's happened ~5 times in the last month.

I don't see any fluid leaking around the master cylinder / booster area. Is this symptomatic of a failing master cylinder (internal leak)? I found this info using Google that seems to support this theory:

http://forums.rennlist.com/rennforums/928-forum/685023-strange-brake-problem-pedal-gets-soft-at-lights.html

http://www.hondaaccordforum.com/forum/general-tech-help-7/1995-accord-brake-pedal-goes-soft-idle-34533/

http://forums.evolutionm.net/evo-tires-wheels-brakes-suspension-sponsored-tire-rack/511555-weird-problem-brake-pedal-softens-hardens-randomly.html

I also get a steering wheel shake on the highway exit ramp (~70 to 20 mph). I understand that this could be due to several things, but maybe it is relevant.

If there are no leaks (and in a month you would have pushed it all out) I would say it's an internal issue with the master cylinder. I would have a local shop or dealer check it out. You don't want to mess around with the brakes.

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cseagle08 07-02-2013 08:17 PM

This is random, but is your a/c on at the same time?

cww 07-02-2013 09:37 PM

Thanks for the replies. I haven't taken the car in for the brake line recall yet. When I do, I'll ask about this. I changed the engine oil tonight, so I had some time under the car to poke around and didn't see any brake fluid leaks.

Quote:

Originally Posted by cseagle08 (Post 4490676)
This is random, but is your a/c on at the same time?

Not sure about the A/C. It's been hot so it's probably been on. I didn't notice this until summer, so maybe the A/C has been on every time. Why do you ask?

ronemus 07-03-2013 08:25 PM

I fixed this on my car by replacing the one-way valve in the vacuum hose between the intake and the brake booster. If the valve doesn't hold vacuum, you get more boost when the RPMs return to low idle (several seconds after you take your foot off the gas).

ragman 07-03-2013 10:37 PM

If its not leaking, another cheap thing to try would be bleeding the brakes with new fluid.

Yeti 08 07-10-2013 10:16 AM

Did you ever figure out what was causing this? Mine is doing the same thing. Pedal gets soft for a second while idling at a stoplight. Only happens about once every other month and I can't duplicate it either. Other than that the brakes are working fine.

ras06LGT 07-10-2013 01:21 PM

I've noticed a similar behavior with my vehicle, albeit the travel isn't as much as you're describing. Mine seems to coincide with the A/C shutting off. My guess is a slight change in available vacuum as the compressor dissengages, increasing the amount of vacuum assist slightly. Haven't noticed it much recently, so I'm not too worried about it.

One test I've done before to track down potential master cylinder issues is to pump the brakes with the car off (to get rid of any vacuum) and then hold down the brake pedal with moderate to heavy force. If the pedal doesn't change after 30 seconds or so, then that's a decent indication of solid MC internals. I was able to diagnose a bad piston seal in my Jeep's rear circuit using this method, as the pedal moved towards the floor after 15 seconds or so of holding the pedal.

Obviously, this is not definative by any means, but if no other leaks are found in the system, it may help to rule things out (or identify them). As stated, don't mess around and take chances with your brake system.

cww 07-11-2013 08:50 AM

No, I haven't figured this out yet. But now that several of you have pointed out the AC / vacuum thing, I think that's the root cause. I've only ever noticed this at idle, during the summer months (AC typically is on). I'll pump the brakes with the car off just to check, and I was planning on changing the brake fluid soon anyway.

ras06LGT, it may be less travel than I'm describing. The resolution of my feet is low. Time for a foot mod lol

FastofWcDave 07-11-2013 08:55 AM

You need to replace your master cylinder. It's leaking internally.


www.FASTofwestchester.com

ras06LGT 07-11-2013 04:50 PM

I just want to say this again, since we are talking about a major safety system here... The test I suggested is NOT a definitive test to determine if there's a problem or not (unless the pedal slowly sinks to the floor - then there IS a problem). It's just a tool to help narrow things down.

Best of luck.

cww 07-11-2013 06:43 PM

ras06LGT, I understand that. Thanks again for your responses.


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