[FORANE]

As long as I have had my 2001 ll bean, the brake booster has been weak. It seems more pronounced in the morning. It seems more pronounced when traveling very slow - like down a driveway slow - and especially when the driveway slopes downhill. I have to apply significant force to the brake pedal - almost as if there is no boost at all. The brakes still work, just little boost. After a minute of driving or less it is all but normal boost as if there is no problem at all.
I replace my own pads, rotors, etc and they are in good condition.
Ideas?

 

[FORANE]

No change if idles for a while, I do not believe anyway. To be clear, the booster is still weak even after car has been driven a while but not very noticeable - I have to be looking for it slowing from faster speeds , on level ground, etc. It just is not hardly noticeable at all above 5 mph and hardly noticeable on level ground. It definitely is more noticeable (read: hard to miss as I press on the brake pedal with much strength) downhill at slow speed like pulling out of my driveway in the morning.

Calipers were able to be C-clamp pushed back without difficulty.

Pads were advance auto but not their cheap ones, they were the upgraded pads and has not changed with the replacement.

 

[FORANE]

OK, More information...
I just got back from a 50 mile trip and for the heck of it decided to drive out my driveway (downhill where it is always the worst); hardly noticeable at all - possibly even normal. Definitely more noticeable when first driven.

checked the brake fluid - just under full and I have never had to add brake fluid.

smoke test?

Would vacuum be lower when cold?

 

[FORANE]

OK
I just did the 15 minute idle then drive down the driveway test. After starting in 54 degree ambient air temp, letting idle for 15 minutes, driving down driveway, brakes seemed just about normal - hardly any weakness in boost - some weakness but very little. I do not know if this improvement in symptoms is due to letting it warm up, or the ambient air temp not being in the 40's, 30's or below, or a combination of the two. Just have not watched it enough to know. It has definitely seemed to be worse with lower ambient air temps in the past.

I am currently away from home with the car and have few tools. So, have not pulled the vacuum line to see if it has vacuum when starting cold.

Will try pumping brakes next time it is cold and see if any effect there.

Don't have equipment to smoke test it.

 

[FORANE]

Sure is sounding like a bad check valve. I will get ahold of some wd40 and try that approach. May take me a few days to get to it but I will report back.

Thanks for the tips guys.

 

[FORANE]

Another story exactly like mine...looks like the check valve is a common failure in our cars.

From here:Subaru Outback Brakes Failed Problems


#5 - Brakes Failed problem of the 2002 Subaru Outback

2002 Subaru Outback, ll bean edition. Brakes have failed to work on several different occasions when outside temperature is less than approx. 28 deg f. This is usually first thing in the morning when leaving for work. Car is parked on driveway with a slight downgrade to the street, and car has drifted into driveway across the street before coming to a stop. Brakes are usually fine after several minutes of driving. This brakes failed issue occurred Tuesday, January 18, 2005.

And here:
http://www.autosafety.org/node/564015

 

[FORANE]

The *frozen* check valve is fairly well known.

I thought your descriptions had been during periods that were too warm- above freezing.

Definitely affected by cold - the colder it is the worse it is. I do not know however that I can state with certainty it does not occur at temps above 32 degrees. Does this absolutely not occur above 32 degrees?

Does anyone know a part number and source for a replacement check valve? If it is cheap I might be inclined to replace rather than try to clean out any gunk built up in there.

 

[FORANE]

ok
I removed the brake booster check valve and sprayed WD40 through it. It seemed to function properly as a 1 way valve as I could only blow 1 direction through it. I reinstalled it and there has been no change. Going down driveway at 35 degrees this am, brakes still a little weak.

I removed the check valve after the car had been sitting for 12 hours and there was no vacuum present. So, I figure it is either a leaking booster or more likely a leaking check valve. I do not have a means of applying a vacuum to the booster for testing its integrity. In either case, the check valve is less expensive and easier to change so I would like to try replacing that to see if it makes a difference.

Can you guys confirm part number compatibility for an 01 outback as referenced in this thread:
http://www.subaruoutback.org/forums/66-problems-maintenance/22468-brake-booster-check-valve.html
It would be a 26151AC110 correct?

 

[FORANE]

I think I found the part number for the brake booster check valve alone without the vacuum hose. It is part number 46403-SE0-003. It is an Alder part also used on acura and nissan. Best of all, it is available on ebay for just $5.65.

 

[FORANE]

Solution!
I can confirm JDM Alder part number 46403-SE0-003 is the exact same part as the brake booster check valve on my 2001 Outback LL Bean.

It also has restored my brake booster function.
Upon inspection of the old and new check valves I noticed the new check valve required very little vacuum to get it to open whereas the old check valve required alot of vacuum before opening like the diaphram was rigid. I believe the old diaphram allowed vacuum to dissipate over night and did not allow vacuum to build up when first started due to its stiffness which increased when cold.

Folks, this is a $5 buck and 1 minute fix. If your booster is weak, especially in the morning and in colder temperatures, look for this part on ebay.