[jason8251]

So as the title says I get air in the brake lines every 6 months or so. We live on a dirt road so the other day when the brake pedal went soft I got going about 25MPH on the dirt and slammed on the brakes to actuate the ABS and the bakes pedal immediately felt solid again. I popped the cap off the master cylinder and there was a big air bubble floating in there. After looking closely at the ABS actuator (I'm guessing that what the ABS module with all the lines in and out is called) I noticed the top of the module where the 4 lines go in there was evidence of some moisture, I'd describe it as "sweating". I probably wouldn't even have noticed if I wasn't specifically looking for a leak on the ABS module and the engine wasn't covered in dust from the dirt road.


I put a wrench on the 4 lines and they're all tight, as tight as I dare to wrench on them, I've cracked enough hydraulic lines in my day to be a little gun shy. Has anyone else had this issue before and if so any ideas how to remedy it.


Thanks fellas

 

[77'911]

You might consider flushing your brake fluid if it has not been done. See if that helps first. It's amazing what some fresh fluid in there will do. Clean any debris around the abs sensor, if there is any. I change our fluid out every 2 years. We wanted a really decent pedal along so I replaced all the pads/rotors all round which really firms up the pedal.
Regards

 

[1 Lucky Texan]

calipers may need cleaning and new grease.

perhaps dust is causing them to bind occasionally?

 

[idosubaru]

Your rear brake lines are leaking above the gas tank.

Splice in new line from under the passengers seat to each rear well, they will blow out for good.

 

[OutbackJeff]

Under the hood on the right side, there is a vacuum line from the engine to the brake booster. They call it the brake booster vacuum line. On the parts break down its labeled: vacuum hose - brake.
On some (not all) Gen 4 years and models, the brake booster vacuum line has a brake booster "check valve" built inside the brake booster vacuum line, that gets stuck.
Pumping the brakes unsticks the brake booster check valve. That's why pumping the brakes restores the brake pedal feel.
If you have the brake vacuum line with the internal check valve built inside, replacing this inexpensive item has a high probability of fixing intermitant soft...or intermittant mushy....or intermitant sinking brake pedal.
If your car doesn't have the vacuum hose with the check valve built into it, then that means the vacuum check valve is built into the actual brake booster (expensive).
Now the bad news. Once the check valve is replaced, within a year or two, carbon particles from the engine vaccum will clog up and stick the replaced brake booster check valve again.

 

[idosubaru]

the check valve is a good call! can't some of them freeze as well if they get too much condensation in the line?

but that's for the brake booster and not the MC. since he claims he saw "a big bubble" in the MC, I'm assuming that's accurate and it's MC/line related.

oh and about the lines - don't touch those without clear and compelling reasons. subaru brake line failure is nearly unheard of. it's either going to leak, or not, and the former is extremely unlikely.

 

[jason8251]

I'll check out the vacuum line when the car gets back this afternoon and see if there's a check valve. Although pumping the brakes hasn't helped before (I think this has happened three times now) I'll replace it before I go tearing into the hydraulics.The fact that there's a little moisture on the ABS module led me down that road, then the bubble floating in the master cylinder after I actuate the ABS and the pedal firming up kinda kept me going that way.


Thanks for the help and I'll keep y'all posted.

 

[jason8251]

Here's some pics, I don't see a check valve in-line but there is either a filter or check valve behind the metal bracket attached to the booster. Is this the check valve you're talking about?

 

[77'911]

When you say "a little moisture on the ABS module", what type of moisture? Is there brake fluid leaking from the abs module? There should be no moisture on the outside of the module.
Regards

 

[jason8251]

Not really sure if it's brake fluid, it's really hard to tell if it's even fluid at all or if it's in any way related. know there shouldn't be any fluid on the outside of the module though. I may be talking myself into something so I can explain the problem. I'm gonna try the check valve and see what happens. The problem is it only happens very so often (it's happened three times in 4 years) it's hard to replicate let alone diagnose the issue.

I still have a hard time wrapping my head around why after I slam on the brakes to get the ABS to activate there's an air bubble floating in the resevoir. I can see the pulsing of the ABS breaking the check valve loose though. We shall see.

 

[idosubaru]

older models you can activate the ABS module - i would want to do that while watching the ABS and MC and see if I can notice anything happening real-time while the ABS is actuating. i would imagine newer cars still retain that ability.

 

[1 Lucky Texan]

when the pedal 'felt solid again' - does that mean the pedal sank very low and activated the ABS, then, after the incident brakes felt normal and were working at the same, higher, pedal position as normal?

 

[jason8251]

idosubaru: How do you go about activating the ABS module without stepping on the pedal? Do you apply 12V somewhere on the module?


Texan you're correct. 3 times in the last 4 years the brake pedal will basically sink to almost the floor. In the past I would bleed the brakes and all would be fine for like 9 months to a year. This last time, about a week ago, I took the car out on the dirt road we live on and slammed on the brakes. I could feel the ABS pulsing and the pedal immediately firmed up back to normal. I got out of the car and took the lid off the master cylinder and saw an air bubble floating in the fluid, that led me down the road of something leaking, or not being tight anyway.