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97 Subaru Legacy LF and RR brakes building up pressure and locking up.


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Thoughtless
New User

May 1, 2016, 5:18 PM

Post #1 of 2 (237 views)
post icon 97 Subaru Legacy LF and RR brakes building up pressure and locking up. Sign In

I have a 97 Subaru Legacy L Wagon with 250,000+ miles that I bought as a fixer upper a few months ago.

It knew it had some brake problems going in, but I have a lot of experience with brakes so I thought it wouldn't be a big issue. After diving in I found multiple slides were locked up on the calipers, they couldn't be freed so I've replaced both rear calipers/brackets and both front brackets. Despite these fixes the brakes would still occasionally act like they were sticking and not long after they started locking up when ever I came to a complete stop.

Since then I've been trying to fix this for about a month now, so let me quickly run through the testing and fixes I've done.

-After locking up if you tap the brake pedal while moving they would often loosen up. (Making me think master cylinder)

-After pumping up the brakes while the car was running I jacked up each wheel individually to see which ones where locking up, from this I found that the LF and the RR where the issue.

-I've checked the flex lines and they aren't collapsing, when opened the caliper would return.

- I traced the steel lines back to the ABS unit (also the ABS light was on, so I thought this might be the culprit) and realized that it was the reason that those specific wheels are locked up because both of those lines run to the same main channel. When opening that main channel, backed up pressure will release allowing the calipers to return. Also recently I completely removed the ABS unit and blew compressed air though it to make sure that flow wasn't restricted or blocked and it wasn't.

-After this I traced that main channel line back to the master cylinder, but when I cracked the line at the MC it didn't release any pressure, which has me really frustrated and confused. At one point the lines travel behind the vacuum booster, but I believe I'm still tracing the correct line.

This leads me to where I am now, my current theory is that the steel line between the ABS and MC is somehow collapsing and acting like a check valve similar to a bad flex line, but I'm not sure if this is even possible and if it is I've never heard of it happening.

Before that I was originally all in on it being the ABS unit, but now that I know it is allowing fluid to flow, the next scapegoat would be the MC and that would of course make plenty of sense (bad cup seals), but there is no pressure build up at the MC from what I can tell (though I will probably test this again tomorrow).

All that I'm left with is the line between the ABS and MC, but there could be something I'm missing. I don't have the money to just throw parts at it and even if I did that's not how I prefer to solve problems anyways. Any help with this problem would be greatly appreciated, because this car has certainly gotten the best of me up to this point.


Discretesignals
Ultimate Carjunky / Moderator
Discretesignals profile image

May 1, 2016, 7:32 PM

Post #2 of 2 (232 views)
Re: 97 Subaru Legacy LF and RR brakes building up pressure and locking up. Sign In

LF and RR are on the same circuit, I believe. If you loosened the line at the master and they didn't release, but releases if you loosen the line after the ABS modulator, that leaves the ABS modulator. Never seen a steel brake line act as a check valve. To test your theory of a collapsed line loosen the line that comes from the master cylinder at the modulator to see if they release.

The test results of blowing air through a modulator and then having thousands of PSI of hydraulic pressure going through it my differ. Another words the modulator may act differently with a fluid than air going through it.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on May 1, 2016, 7:37 PM)






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