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2001 OBW VDC, 2004 WRX, 1999 LEGACY L (in a heap)
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Discussion Starter #1
2000 2.5 auto

Okay, so random pedal dropping with no sign of fluid leaks or pressure loss, I've been suggested, is likely an ABS failure.

How do you test and ABS system for this? That information seems to be a bit scant in favor of parts shotgunning.
 

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2001 VDC/SC One of a Kind
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It's more than likely your master cylinder not the ABS unit.

What do you mean by no apparent signs of pressure loss and the pedal is going to the floor.

The ABS unit is tested with the software that communicates with the ABS.
 

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2001 OBW VDC, 2004 WRX, 1999 LEGACY L (in a heap)
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Discussion Starter #3
(this is a 3rd party diagnostic effort)

Master cylinder has been shotgunned. problem persists. And there is no loss of fluid in the system to accompany the pedal sinking, the reservoir was still topped, and when bled the fluid was right there without new air entering the system, so it seems like no pressure loss.
 

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2001 VDC/SC One of a Kind
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When a master cylinder is failing it is generally because the relief valve in the master cylinder let's the fluid return to the reservoir rather than applying pressure to the system.

Usually with abs issues it's a relay fault, not hydraulic. Relay faults generally come up when the battery is weak or ground is bad.
 

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Discussion Starter #6
meaning they have already swapped out the master cylinder and symptoms persist.
 

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Discussion Starter #8
When a master cylinder is failing it is generally because the relief valve in the master cylinder let's the fluid return to the reservoir rather than applying pressure to the system.

Usually with abs issues it's a relay fault, not hydraulic. Relay faults generally come up when the battery is weak or ground is bad.
ok, thanks. Is there a diagnostic guide for those two possibilities? How do you test a relief valve, and how do you check the abs relay function?
 

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2001 OBW VDC, 2004 WRX, 1999 LEGACY L (in a heap)
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Discussion Starter #9
Did you put a brand new one on or a remanufactured one? Remanufactured master cylinders are junk.
details take a moment to filter back to me. Looks like they used a red/green store's master cylinder. As crumby as those are, i am highly doubtful they would immediately exhibit the same symptoms upon installation, but if there's actual diagnostic method, i'm ready to learn
 

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2001 VDC/SC One of a Kind
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You'd be surprised how fast a reman master cylinder out of the box and on the car will be crap. Like as soon as you put fluid in it and bleed It out crap.

When they rebuild a master cylinder they hone out the cylinder bore and put oversized seals in it. That doesn't work because you're changing the physics of the master cylinder which changes the pressure to the system.

Plus they tend to leak back.
 

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2001 VDC/SC One of a Kind
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You raise the car up, clamp off the brake hoses at the calipers, then you get in the car and push down on the brake pedal and see if it goes to the floor. Start the car and see if it goes to the floor. If it does the master cylinder is bad. Highly doubtful there's anything wrong with the ABS hydraulics.
 

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The old skool quick-n-dirty check was to open the master cylinder cover and have someone depress the pedal slowly. The fluid would percolate if the seals were still good to push fluid. If nothing moved, we'd change it out.
I guess these new fangled m/c have some extra voodoo in them that makes this method inconclusive.

Does the pedal build any pressure or height when pumped?
If it does, I'd say the new m/c was not bled thoroughly and there is trapped air in the system.
 

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2001 OBW VDC, 2004 WRX, 1999 LEGACY L (in a heap)
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Discussion Starter #13
The problem was described as intermittent. Sometimes there would be brakes, sometimes the pedal just sunk.
 

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Another way is the exact behavior. You would probably recognize the behavior if there's air in the lines - the pedal is immediately soft/mushy and you can usually pump it up to become hard.

A failing (bypassing) master cylinder usually has a nice hard pedal when you first press. Then if you're holding the brakes like you would at a red light, it slowly and smoothly sinks to the floor. For me it has been especially when the engine compartment has got nice & hot (like running the AC or idling).
 

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2001 Outback VDC Wagon
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I am having a similar issue (pedal is firm at first, but will slowly drop with a decent amount of continuous pressure) with my 2001 Outback VDC wagon. Got an OEM master cylinder and a used (but 45,000 less miles, so "new") ABS pump for the VDC model and replacing both this week as a 4th of July project. I will let you know of any caveats or issues.

I will second that non-OEM ones can be junk, straight from the box. Happened to me on a different car (1987 Dodge RAM B250), and I wasted so much time I just get OEM and consider the extra cost paying myself back for any time dealing with it 😅
 

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Discussion Starter #18
Thx all. updating...

...brake hoses at the calipers, then you ...
This was already done apparently to look for leaks and air getting into system.(Neither of those were found.) Pedal does sink, but again its not predictable. Most presses are firm, but they were before the mc swap too.

I confirmed the replacement was a unit from the green O, and it was the one described as new parts, not remanu. They do make a distinction between them on the UPC description, so when I get the brand, if it is still the MC i'll let you all know whom.

I'm still wanting to eliminate other explanations to help just buying another MC or random parts until something works, if i can help it. This is their primary daily vehicle, the money maker.

So any other reasons for pedal drop? And what would the diagnostic for it be?
 

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2017 Outback Premium 2nd Subaru owned
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I had a 2003 OB that had the same problem. After replacing the master and vacuum booster with a New Subaru unit, the problem still happened. There was a recall on the ABS system. My car was not covered just outside the recall date. BUT the part that was on the car was part of the recall. Subaru would not do anything as the car was off the extended warranty. What we did was replace the ABS unit with a low mileage 2005 OB unit. Problem solved.
 

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2017 2.5i Limited SSD Strut Brace
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Any exhaust work done or a brake line that has gotten too close to the exhaust system and causing the brake fluid to boil?
 
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