Low Brake Pedal - master cylinder?

[vasy]

96 Outback wagon, 139,xxx miles. After a 60-mile drive at highway speed today, I stopped at a bank parking lot. Started the car 5 minutes later, while backing out of the parking space, the brake pedal went unusually low. It happened twice within 30 seconds in the same parking lot but never happened again. This is the first time I've ever experienced this in this car in 17 years. I know this car's history and there has been no problems with the brakes. Brake fluid is full and was replaced (by flushing) last year by myself. I really don't think there is air trapped in the system. Is the master cylinder the only thing to blame? Could a faulty brake booster cause a low pedal like this? I know master cylinder is the #1 culprit but would like to know that for sure before buying the part. It's odd that it happened right after starting a hot engine. That's why I suspect the booster - might not have enough vacuum right after engine start.

[idosubaru]

Subaru MC failure is almost unheard of.  It's easy to find folks that try to replace it but then that's not the issue (been there done that, here's someone else that did it today:

http://www.subaruoutback.org/forums/67-classifieds/79818-fs-brake-master-cylinder-new.html)

 

what is more common is a leak, but don't think spraying, spewing.  think initial start of one, like the metal is still covered but sometimes it can squish through cracks in the now rusty pipe.  when they first start to leak it's a very slow seapage and random, that is just enough to cause give in the system but not show any fluid loss.  the fluid lines that snake up above the rear crossmember and gas tanks can leak slowly, like a drop at a time (every time you feel that pedal go lower than normal), and not leak enough to run down over stuff and finally become visible from underneath.

 

or it can leak past the piston seals and still be contained behind the dust boots on the caliper.

 

i've seen both.  in time they'll really fail as the hydraulic pressure works on that compromised area and blows it out for good.

i've never seen a master cylinder failure on a Subaru and it's rarely seen on forums, but of course it is possible.

[MilesFox]

My 95 legacy wagon failed the MC at 245,000 mi.

 

I believe one channel has been failed for a while, as the front left wheel would lock up in winter driving, and then finally brake pedals would go to the floor. Now that i have replaced it, the ABS module is leaking.

 

I wouldn't rule it out for a 15-20 yr old car.

 

Aside from subarus, the MC failed in my 78 toyota chinook camper (1/2 ton) with 300,000 mi on the clock. My 81 ford ambulance is intermittent with brake pressure with the MC, although i have fixed all the leaks. Time for a MC on that one, too.

Edited July 23, 2013 by MilesFox

[Fairtax4me]

Booster would not cause a low pedal. A failed booster would leak vacuum causing a stiff pedal.

 

I agree with Gary, start looking for leaks. Especially around the rear cross-member and where the lines run above the fuel tank.

[gravitate]

I had this happen to me. First time I replaced the master cylinder and and it didn't fix it. It kept happening over and over but mostly when I was pulling out of a parking spot and sometimes at a trafic light. After replacing everything I could think of I finally figured out that I had a loose wheel bearing. I had no noise or wobble coming from it so thats what really confused me. When I would put on the brake it would straighten the wheel up vertically. When I would take my foot off the brake it would allow the wheel to tilt ever so slightly to the side causing the caliper to expand so that when I would put my foot on the brake again the pedal would go down a lot further than normal to push out the caliper till the wheel would straighten up again. Just a thought.

[vasy]

I had this happen to me. First time I replaced the master cylinder and and it didn't fix it. It kept happening over and over but mostly when I was pulling out of a parking spot and sometimes at a trafic light. After replacing everything I could think of I finally figured out that I had a loose wheel bearing. I had no noise or wobble coming from it so thats what really confused me. When I would put on the brake it would straighten the wheel up vertically. When I would take my foot off the brake it would allow the wheel to tilt ever so slightly to the side causing the caliper to expand so that when I would put my foot on the brake again the pedal would go down a lot further than normal to push out the caliper till the wheel would straighten up again. Just a thought.

Wow that's something new to me. I'm glad you replied. I haven't checked but I don't think my metal brake lines are rusted to the point of leaking. There is very little rust on my car's undercarriage. So your experience can help me. A few questions to follow up:

1. Was your loose bearing on the front wheel?

2. Why was your bearing loose, because the axle nut was loose?

3. At the moment you had this problem, were you turning the steering wheel at all, or were you pulling out of a parking lot straight without turning the wheel? When it happend to me yesterday, I was not turning the steering wheel at all. Simply slowly and straightly backing up in a parking lot.

 

I'll put the car on jack stands and do a thorough check tomorrow: leakage and bearing. When I removed the snow tires a few months ago, I did grasp both sides of the front tire (off the ground) and tried to rock it. No movement. It's only been about 1000 miles since then.

Edited July 24, 2013 by vasy

[gravitate]

It first happened when I was backing out of a parking lot, wheel was not turned at all. It would happen with the wheel turned or straight though but more when it was straight. I think thats because when the wheel is turned it is already cocked over and the caliper piston is already spread out but when it's straight and the wheel leans over then the piston expands and therefore you notice it because it has to compress thus making the pedal go down further.   The bearing just started going out from normal use I guess, the axle nut was tight. This happened to me with both the front and the back. I'm supprised I have never heard of this happening to other people when it's happened to me twice.

[vasy]

It first happened when I was backing out of a parking lot, wheel was not turned at all. It would happen with the wheel turned or straight though but more when it was straight. I think thats because when the wheel is turned it is already cocked over and the caliper piston is already spread out but when it's straight and the wheel leans over then the piston expands and therefore you notice it because it has to compress thus making the pedal go down further.   The bearing just started going out from normal use I guess, the axle nut was tight. This happened to me with both the front and the back. I'm supprised I have never heard of this happening to other people when it's happened to me twice.

Did this ever happen to you when you were driving at higher than parking lot speed? If it only happens at lower speed, I can take my time to get it fixed.

Did you say both your front and rear bearings caused this low-pedal problem and once bad bearing is replaced, the problem is gone? Thanks.

[gravitate]

I definitely noticed it at low speeds more. Once I replaced the bearings the problem was fixed. One bit of caution. I had this happen to me once when my outer cv joint came apart, I'm not sure how that caused the same problem but it did. My brake pedal went straight to the floor and had I not had a ebrake I would have crashed into the person in front of me. My point is, there is only a certain distance one stroke of the brake pedal can move the caliper so if the caliper gets spread to far your not stopping unless you pump it so better to replace now then wait.

[vasy]

I thoroughly checked for brake fluid leakage today, none was found. Axle nuts were tight. I also held the tire by both hands and tried to move it back and forth. For both rear tires, no movement. The front tires both had a little movement in the 3/9 o'clock position, but the movement is tiny, barely noticeable. None at the 12/6 o'clock position. Other than that, I don't know how to check for bad/loose bearings. Spinning the rear tires got me the brake pads friction noise. Can't hear anything else. Didn't bother removing the brake pads and then spin the tire. I definitely didn't hear any noise when driving the car (with the radio off). I'll drive the car tomorrow to see if the low pedal happens again. It's really odd if bad wheel bearings can give low brake pedal, it doesn't happen to most cars with bad bearings. I'm now thinking 50% master cylinder, 50% bad bearing. There is no leak I can see. The brake fluid level in the reservoir hasn't changed at all, remaining a tad above the max line.

[1 Lucky Texan]

the bearings would have to be monumentally bad.

 

should be easy enough to put a vacuum gauge on the car just to confirn it's OK.

 

There is a vacuum check valve in the vacuum line going to the booster. i have read of them getting moisture and freezing in the winter. I suppose it's possible they could get gummy or otherwise fail. they are supposed to maintain enough vacuum in the booster so you have a coupla easy stops if there's an engine failure on the road.

 

I'd say there's a good chance you lost booster vacuum after you parked the car. Not sure why though. Booster leak or check-valve problem seems likely though.

Edited July 25, 2013 by 1 Lucky Texan

[hankosolder2]

A lack of boost will cause a firm, high pedal, not extra pedal travel- assuming that you're describing the problem accurately. A momentary loss of boost could cause a firm high pedal, and then when the boost returned, the pedal would feel like it was sinking (down to the normal position it would be in for that given amount of pedal pressure) due to the assist- but a booster can't increase the pedal travel by itself.