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.

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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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.

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?

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.

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

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

http://www.FASTofwestchester.com

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.

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.

Long term update: I believe ras was correct. It's been over a year and I've been able to experience this many times, and have noted that it occurs when the AC compressor turns on or off.

Simple test: while the vehicle is running and parked, apply moderate force to the brake pedal, then turn on your windshield defroster (max fan, AC on) and pay attention to the pedal feeling as you hear the HVAC kick on. I was able to consistently replicate this slight pedal sink under these conditions.

As ras said previously, don't mess around and take chances with your brakes. If you think something is wrong, see a professional.

I can't put it any better than Honda. Summary: This is normal, do not adjust your set.

This change in brake pedal height is caused by an increase in engine vacuum. The "power brakes" system in most modern vehicles uses engine vacuum to boost the amount of force applied to the brake pedal by the driver of the car. The device that does this, called the brake "booster", makes it easier for the driver to stop the vehicle. Engine vacuum is affected by many different conditions. Engine vacuum is highest when all accessories are off and the accelerator pedal is released (low load), and lowest when all accessories are turned on and the accelerator pedal is held to the floor (high load). Any change in engine vacuum (load) can affect the amount of "assist," or boost, provided by the brake booster. One system that applies a noticeable load to the engine is the air conditioning. During normal operation, the air conditioning system constantly cycles on and off, changing the amount of load on the engine. When the air conditioning cycles off, the load on the engine is decreased and the amount of engine vacuum is increased. If the driver is applying the brake pedal when the air conditioning cycles off, the increase in engine vacuum increases the amount of boost applied to the brake pedal. That increase is felt as a slight drop in brake pedal height and is a normal characteristic.

……...When the air conditioning cycles off, the load on the engine is decreased and the amount of engine vacuum is increased. If the driver is applying the brake pedal when the air conditioning cycles off, the increase in engine vacuum increases the amount of boost applied to the brake pedal. That increase is felt as a slight drop in brake pedal height and is a normal characteristic.

Exactly what I have been feeling for years. Thanks for the info.

Long term update: I believe ras was correct. It's been over a year and I've been able to experience this many times, and have noted that it occurs when the AC compressor turns on or off.

 

Simple test: while the vehicle is running and parked, apply moderate force to the brake pedal, then turn on your windshield defroster (max fan, AC on) and pay attention to the pedal feeling as you hear the HVAC kick on. I was able to consistently replicate this slight pedal sink under these conditions.

 

As ras said previously, don't mess around and take chances with your brakes. If you think something is wrong, see a professional.

Glad you got it sorted out.

We've had the very same issue. It's been to the dealer, they advised us it was normal . The service mngr. advised us if pedal sinks to floor when applying pressure at a stoplight to give it a little gas which will increase the vacuum to bring the pedal back up. Since when on a brand new car or any car at that do you need to drive with two feet, on an automatic so I have brake pressure. He's got jokes for days!

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Well since I'm a girl, I know nothing an I should just believe his shit. However I'm not bragging, but I'm very knowledgeable about cars. I most certainly know that this isn't right an I wasn't afraid to put this so called "Subie Service Mngr." in his place.

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Well since I'm a girl, I know nothing an I should just believe his shit. However I'm not bragging, but I'm very knowledgeable about cars. I most certainly know that this isn't right an I wasn't afraid to put this so called "Subie Service Mngr." in his place.

 

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Good stuff. Those type of people, who are paid to make sure complex machines (cars) are in good running order, but don't, shouldn't be allowed anywhere near any machinery (maybe bricklaying is a better career for them?).

I just experienced this problem yesterday. I have 80,000 miles on my 2013, and for perspective, I am still on my original pads and rotors, as I do mostly highway and I am VERY easy on my brakes. I also flush my brake fluid almost once every 12-18 months. (I've done it about 4 times since new)

Traffic on the interstate came to a stop, and while sitting stopped with my foot on the brake, the car was creeping forward and I had to push the pedal to the FLOOR to keep it stopped. VERY scary. I found that "pumping" the pedal helped.

I was 150 miles from home on a business trip so I googled the closest Subaru dealer and found one 6 miles away! I drove VERY carefully those 6 miles to the dealer

After 1 hour of troubleshooting, they determined a bad master cylinder. $450 to replace, PLUS they wanted $149 for a brake system flush.

I told them that the brake system flush is part of replacing the master cylinder and after a few minutes of me describing the scope of work for each, the service guy gave up and "agreed to cover that work, no charge". After searching 6 dealer website for service specials, I also found a 10% off coupon as the most effective. I was hoping to find better.

Doesn't it seem strange for a brake master cylinder to fail on a 2013 with 80,000 miles, all highway and minimal brake use?

Should I call Subaru and try to get some goodwill?

Any other thoughts/comments/suggestions?

I had my 91 Prelude for over 17 years and 200,000 miles and it had the original master cylinder and awesome 4 wheel disc brakes with no issues. This is a surprise...