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01 LL Bean
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Discussion Starter · #1 ·
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?
 

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03 H6 OBW & 06 WRX Sportwagon
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weird

if you were to let the car idle for 10-15 minutes before moving it, would the brakes feel normal?
 

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Somebody Else's XT
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Maybe your booster is fine, and your pads don't grab well until they've got some friction heat?

I put more aggressive pads in mine and this effect became more pronounced vs. stock.
 

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03 H6 OBW & 06 WRX Sportwagon
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I definitely did not do myself any favors buying pads on sale once, they were horrible.

so called 'economy' pads either won't last, or won't stop the car. There are also some high performance pads that won't grab at cool temps.
 

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2003 LL Bean Outback H6 and 2019 Outback Base
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514 Posts
how are the calipers ?.....my old one had similar issues and one caliper was semi frozen
 

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01 LL Bean
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Discussion Starter · #6 ·
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.
 

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03 H6 OBW & 06 WRX Sportwagon
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not saying it isn't the booster, but that would be on/near the bottom of the list of suspects. Vacuum would be highest at idle and even if the check valve were bad, I'd expect idling would both thaw it out/loosen it AND offer plenty of time for any 'lost' vacuum to build.

While the pistons may move freely, there could be an issue with the slide pins not allowing the caliper to move freely.

And it is probably worth trying to bleed the system again, though I might not understand how that would change with time/temperature.

have you checked the brake fluid level? If it somehow were leaking onto the rotor, you may feel decreased braking until it burned off. Same might true for rust or even dew.

I might be tempted to try a bedding in procedure. At least do the '30 medium/hard stops from 30mph with 30 seconds off-brake cooling between each stop' trick. There are more aggressive bed-in procedures but you can search for those.
 

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06 OBW 2.5, 05 Forester, had 03 H6 OBW
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I think my booster might be weak, just as described. Bought the car with 55K on it, always took a bit more force on the brake pedal than any car I've driven since my old pontiac with 4 non-power drums off a single-action master cylinder.

I know all 4 calipers and all 8 sliders are free, and I have had a mix bag of rotors and pads on it over the past 145,000 miles.

It hasn't gotten worse, except of course when the check-valve freezes, so I've just been living with it.
 

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03 H6 OBW & 06 WRX Sportwagon
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ours 'may' have slightly more travel in the brake pedal than other cars I've driven, but not much. But it is consistent from the first stop to the last.

I wonder what type of tires Forane is running? that can make a difference.
 

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01 Outback LL Bean
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Do a smoke test and see if the booster is leaking anywhere. You could also apply a vacuum and see how long it holds.
 

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01 LL Bean
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Discussion Starter · #11 ·
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?
 

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01 Outback LL Bean
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With a smoke test you pump smoke into the booster and if there are leaks it will leak out around the booster. A smoke test is a reliable way for locating vacuum leaks.

There is a one way valve that is used to hold vacuum as well, that could be damaged, but it wouldn't explain why the issue is less noticeable when the car is driving. Vacuum is highest when the throttle plate is the most closed.

Foot off the gas = throttle closed = high vacuum
Pedal to the floor = throttle all the way open = low vacuum.
 

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03 H6 OBW & 06 WRX Sportwagon
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Would vacuum be lower when cold?
yes, due to the higher revs/slightly more open throttle/IACV w'ever. vacuum would be reduced.

That's why I wondered if, say, first thing on a cold morning under conditions which you feel the car would exhibit poor initial braking (*breath!*) you instead, allow the car to warm-up for 15 minutes before moving it. The revs should have come back down and the engine should be warm at that point and have had an opportunity to build vacuum.

Then monitor the performance when you pull away. If it fails, the problem is less likely to be the booster because, vacuum should have had plenty of time to build. If braking is normal, the problem is more likely to be the booster.

I dunno, maybe I'm not thinking right about this.

The booster, with a functioning check valve, 'should' hold a vacuum for a significant amount of time. It's designed to provide 1 or2 'boosted' brake applications should the engine die on the road. I suppose you could pull the hose off it in the morning to see if you hear air rush in or not.
 

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'03 Outback H6-3.0 Black Granite Pearl, base model with cold weather package and cloth seats.
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How about just pumping the brakes a few times before setting off for the first drive? Get everything loosened up and moving.....
 

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01 LL Bean
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Discussion Starter · #15 ·
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.
 

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03 H6 OBW & 06 WRX Sportwagon
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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..

I guess I wasn't clear but, I was wondering if the car would maintain a vacuum overnight so, pull the hose before starting it and monitor for air rushing in.

I dunno, not sure if a car should hold that vacuum overnight or not. I know I've read about issues like this, but I'm fuzzy on the troubleshooting.

maybe read here; http://www.subaruoutback.org/forums...en-check-valve-brake-booster-vacuum-line.html

Brake booster not working when cold outside? - 1990 to Present Legacy, Impreza, Outback, Forester, Baja, WRX&WrxSTI, SVX - Ultimate Subaru Message Board
 

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01 Outback LL Bean
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Or you could attempt to apply the breaks and see if you have boost after letting the car sit overnight but before starting it.
 

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01 LL Bean
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Discussion Starter · #18 ·
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.
 

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01 LL Bean
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Discussion Starter · #19 ·
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
 

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06 OBW 2.5, 05 Forester, had 03 H6 OBW
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The *frozen* check valve is fairly well known.

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