idosubaru Posted April 8, 2010 Share Posted April 8, 2010 (edited) Friend said her legacy brakes weren't working at all, had to use the emergency brake to stop. 1996 Legacy L wagon automatic 4WD. I found no leaks, master cylinder has fluid, and it seemed to work fine in the parking lot where I work. The brake light was on a couple days ago and I wiggled the fluid level indicator in the master cylinder and the brake light went out so I figured it was just a sticking indicator. So what can "fail intermittently" so to speak? Edited April 8, 2010 by grossgary Quote Link to comment Share on other sites More sharing options...
Legacy777 Posted April 8, 2010 Share Posted April 8, 2010 She sure the brake booster/assist just didn't stop working? Did the brake pedal go to the floor when the brakes stopped working? If so, did she try and pump the brake pedal? Or was the brake pedal just stiff and not stopping the car? Quote Link to comment Share on other sites More sharing options...
Rooster2 Posted April 8, 2010 Share Posted April 8, 2010 I would think that you have checked the brake fluid level in the reservoir to know that it is full. I suspect that the master cylinder is going bad. Quote Link to comment Share on other sites More sharing options...
CNY_Dave Posted April 8, 2010 Share Posted April 8, 2010 Could even be a bad wheel bearing pushing the pad back into the caliper, this one's wide open. What the pedal did is key, as mentioned. Dave Quote Link to comment Share on other sites More sharing options...
WoodsWagon Posted April 9, 2010 Share Posted April 9, 2010 Does the car have ABS? Quote Link to comment Share on other sites More sharing options...
idosubaru Posted April 9, 2010 Author Share Posted April 9, 2010 She said the brake pedal hit the floor. Based on the replies I'm thinking master cylinder since the "Brake" light came on too? Seems awfully suspicious for the brake light to be on (reservoir full) and then this happens a day or two later. I wasn't sure what failures modes the proportioning valves or brake booster have, never seen or heard of those failing. For her sake I don't want to spend any more money than we have to in order to fix it (labor being free it's parts only). No ABS. Quote Link to comment Share on other sites More sharing options...
lmdew Posted April 9, 2010 Share Posted April 9, 2010 I'd do a complete bleed and see what the results are. I failed master cylinder will show up for sure if you are manually bleeding the brakes. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted April 9, 2010 Author Share Posted April 9, 2010 I saw online where excess moisture in the lines can cause the fluid to boil and fail like this as well - there was a name for it. Anyway, seems unlikely given the light kind of driving she was doing. But it was an odd day and 90 degrees when it happened. There is all sorts of dirt in the MC and I had to rebuild a front caliper on this car less than a year ago. I'm betting the fluid was never changed for 15 years. I just hate "not knowing". The manual bleeding sounds like a great idea, I'm almost hopeful the master cylinder is bad and I get to see it. How exactly do MC's fail and why would it show up one time only? Quote Link to comment Share on other sites More sharing options...
hankosolder2 Posted April 9, 2010 Share Posted April 9, 2010 I agree- most likely a failing M/C, but as others have noted, it could also be if something caused a pad to be pushed back...also if a pad or caliper sticks, it can boil the fluid...did she notice any funky smells? re: failure mode of M/Cs... sometimes they will have a bypass leak under light pedal pressure and be OK under firm pedal pressure. I think that the seals are pressed more firmly up against the bore under hard pedal pressure, so if the seals are marginal, they can have a bypass leak under some conditions and not others. Quote Link to comment Share on other sites More sharing options...
Legacy777 Posted April 9, 2010 Share Posted April 9, 2010 I'd still try a full bleed of the brake system. The prop valves don't typ fail....I've never had one fail. The MC, the seals could be torn up, or get folded over. Here's pics of one I took apart. http://main.experiencetherave.com/subaru/images/mastercyl/ If there was some gunk in the MC, it could have gotten trapped between the bore and seal and caused the fluid to leak past it. You could try opening up the MC and inspecting/cleaning everything. Unless a seal is worn or the bore is scored, you may be able to salvage the existing MC. If you do bleed the brakes, take a turkey baster and suck out all the old fluid in the MC so it doesn't get pushed further into the brake system. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted May 18, 2010 Author Share Posted May 18, 2010 I have another friends Legacy with questionable brakes. 2002 Legacy Wagon GT. 265,000 miles. Brakes work great...then randomly the pedal will occasionally slowly sink to the floor, like every 5 or 10 times trying to stop. I still haven't fixed the first car I started this thread about - wrong M/C, then it was ordered, they didn't get it, couldn't get it, finally ordered online and waiting for it. If the fluid was old it wouldn't do that would it....work perfect, brakes feel great, then once in a while gradually sink to the floor? Not that I'm against flushing fluid, but this is serious too. I'm planning on replacing the M/C in both just to be sure. Quote Link to comment Share on other sites More sharing options...
Fairtax4me Posted May 18, 2010 Share Posted May 18, 2010 Had a friend who JUST had the same problem in a BMW 325. Pedal would randomly, slowly sink, and the car would start rolling (if on a hill). Pump the pedal and it would come right back up. She dealt with it for a while because it would only do it once every few weeks. Eventually got to the point where it was doing it once a day. Replaced the MC, problem solved. Or so it would seem. It has only been 4 days. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted May 18, 2010 Author Share Posted May 18, 2010 Great thanks FT! I was hoping to hear someone with some similar experience. Don't really want to give a friend their car back without having a good feeling it's fixed. Quote Link to comment Share on other sites More sharing options...
john in KY Posted May 19, 2010 Share Posted May 19, 2010 If the MC is full and the pedal goes to the floor, has to be the MC if there is no air in the system. Quote Link to comment Share on other sites More sharing options...
Legacy777 Posted May 19, 2010 Share Posted May 19, 2010 Yeah....I'd agree. Based on the symptoms it sounds like the MC. There may be some trash in the MC resevoir that has made it into the MC and is sometimes interfering with the seal. Over time this will wear the seals down in the MC and cause the problem all the time. This is a good example of why it's important to change your brake fluid annually so dirt and moisture suspended in the brake fluid does not precipitate out as the brown nasty "junk" sometimes found on the bottom of MC resevoirs that have not had fluid changes in a while. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted May 19, 2010 Author Share Posted May 19, 2010 If the MC is full and the pedal goes to the floor, has to be the MC if there is no air in the system.Great, thanks john. Installed the MC, get some time later to bleed it. This is a good example of why it's important to change your brake fluid annually so dirt and moisture Oh man was it dirty reservoir and fluid. Only a 2002 but I guess with 265,000 miles it's seen plenty of use, my buddy commutes like 200+ miles a day in this thing. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted August 12, 2010 Author Share Posted August 12, 2010 turned out to be rear metal brake line leaking due to rust. i think that i wasn't able to experience the issue made it hard to track down. drove it home and the leak apparently got really bad, making it easy to find. Quote Link to comment Share on other sites More sharing options...
Fairtax4me Posted August 13, 2010 Share Posted August 13, 2010 turned out to be rear metal brake line leaking due to rust. i think that i wasn't able to experience the issue made it hard to track down. drove it home and the leak apparently got really bad, making it easy to find. Well.... I suppose that could cause it too... Quote Link to comment Share on other sites More sharing options...
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