Intermittent loss of brakes - Subaru Outback - Subaru Outback Forums
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post #1 of 18 (permalink) Old 04-15-2019, 04:28 PM Thread Starter
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Intermittent loss of brakes

This is a problem on my sister's 07 Ll bean that has me stumped. I haven't experienced this first hand, but here is what I have gathered from what she told me.

When breaking, the pedal will at random times go all the way to the floor and provide no braking force. This seems to happen mostly when the car is first driven after sitting for a few hours (but not always) and has also occurred on a few occasions when breaking down a long hill (less common). In these events, pedal return is very slow, but pumping seems to help regain force. Brakes will then return to perfectly normal condition with good pedal feel. No ABS light on, No ABS codes, no noises, no obvious leaks or loss of fluid. 167k w/ very good maintenance.

I bled the brakes a few weeks ago with a motive bleeder (w/ ignition on) thinking it was an air issue, possibly a bubble trapped in the ABS. This firmed up the pedal a bit, but about a week later same problem occurred. I have driven the car and not been able to replicate this issue. My brother in law can confirm this random failure has happened to him and otherwise the brakes feel good and work as they always have. In looking at the MC when pressing the pedal all looks normal.

My best guess is this is an ABS issue, but I'm not sure where to go next in diagnostics. Any ideas?

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post #2 of 18 (permalink) Old 04-15-2019, 04:43 PM
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I would also check the brake booster and it's vacuum line from the manifold to ensure the power brake system is working as intended. A faulty check valve to the booster or a leak around the diaphragm could cause a spongy or irregular pedal feel.

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post #3 of 18 (permalink) Old 04-15-2019, 05:26 PM
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That's an obviously dangerous situation. My suggestion would be to take the car (better, have it towed) to a competent mechanic immediately.

Don't mess around guessing about what's wrong with a brake system that obviously has something seriously wrong with it. Even if you have a working hand brake.
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post #4 of 18 (permalink) Old 04-15-2019, 06:34 PM
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There seems to be TWO possible symptoms here

1) Pedal goes to floor
-or-
2) Pedal gets 'hard' (looses power)

#1 above is nearly impossible to happen because there are TWO separate hydraulic systems. (redundancy).... one could spring a leak...and two wheels would still have braking-power.

#2 is most-likely the vacuum-booster... There *is* braking available, but it requires a lot of pedal-pressure.
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post #5 of 18 (permalink) Old 04-15-2019, 07:28 PM
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I agree with @brucep, but if the poster can't distinguish between those cases, he should leave the job to a pro.
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post #6 of 18 (permalink) Old 04-15-2019, 07:40 PM
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I would check the hose that goes from the brake booster to the intake where is pulls it's vacuum. There is a really sh*tty spring clamp on the intake side. I bet $5 is leaking there. Put a worm clamp on that connection and see if things change.

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post #7 of 18 (permalink) Old 04-16-2019, 08:57 AM
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It's unlikely to be the ABS. Subaru brake systems are robust and have few issues - this makes diagnosis tricky and guesswork of limited efficacy.

I would *very carefully* monitor fluid level and look under the car with a light and careful attention - following every line from the master cylinder all the way to each caliper. Look for a leak. It doesn't take much loss to cause issues. Even a tiny leak that's almost imperceptible in the master cylinder reservoir, and doesn't drip on the ground is tremendously problematic.

This happens on vehicles with rusty brake lines. The rust "scales" the metal lines and it doesn't cause a hole exactly - but many layers of scaled rust. It's very interesting looking - with no holes at all the fluid just seeps through like a maze through the various layers of rust. And it can be intermittent. Ambient conditions, rust progression, brake use - can cause it to normally work fine but sometimes find it's way through the right combination or layered rust maze.

The most common ones to cause this are the ones above the gas tank - they're exposed, never washed, can't be seen. The fluid very slowly just wets the metal lines/saturates the rust and doesn't drip. or it drips and just solidifies with 10 years of dust/debris on top of the gas tank and never pools enough to run down the gas tank and be visible.

But this is usually 1990's Subaru's with rust....not 2007's....but I can't see the vehicle so I dont' know what we're dealing with. This is the most common causative issue for what you're describing but I'm doubtful this is the case here due to how new this car is. If it was a 1995 Subaru in Maine I would almost bet money that's what it is.


1. Can you confirm there are no lights on the dash and haven't been for quite some time?
2. Has this car every been wrecked?
3. Has this car ever seen a major repair?
4. Is there any rust under the vehicle?


Quote:
Originally Posted by skipw View Post
I agree with @brucep, but if the poster can't distinguish between those cases, he should leave the job to a pro.
This isn't always that easy on asymptomatic vehicles. how can they diagnose something that doesn't exist? They usually replace by guesswork and get it wrong the first...or third...time.
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post #8 of 18 (permalink) Old 04-16-2019, 09:03 AM Thread Starter
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I'll for sure double check all the vac lines and check valves to the booster. From my experience booster issue = hard pedal, and in this case a super soft pedal seems to be the issue (hence the head scratching).

The car is obviously not being driven. I'm going to be working on it this weekend and trying to make game plan / order parts.
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post #9 of 18 (permalink) Old 04-16-2019, 09:13 AM
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post #10 of 18 (permalink) Old 04-16-2019, 09:18 AM Thread Starter
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Quote:
Originally Posted by idosubaru View Post
It's unlikely to be the ABS. Subaru brake systems are robust and have few issues - this makes diagnosis tricky and guesswork of limited efficacy.

I would *very carefully* monitor fluid level and look under the car with a light and careful attention - following every line from the master cylinder all the way to each caliper. Look for a leak. It doesn't take much loss to cause issues. Even a tiny leak that's almost imperceptible in the master cylinder reservoir, and doesn't drip on the ground is tremendously problematic.

This happens on vehicles with rusty brake lines. The rust "scales" the metal lines and it doesn't cause a hole exactly - but many layers of scaled rust. It's very interesting looking - with no holes at all the fluid just seeps through like a maze through the various layers of rust. And it can be intermittent. Ambient conditions, rust progression, brake use - can cause it to normally work fine but sometimes find it's way through the right combination or layered rust maze.

The most common ones to cause this are the ones above the gas tank - they're exposed, never washed, can't be seen. The fluid very slowly just wets the metal lines/saturates the rust and doesn't drip. or it drips and just solidifies with 10 years of dust/debris on top of the gas tank and never pools enough to run down the gas tank and be visible.

But this is usually 1990's Subaru's with rust....not 2007's....but I can't see the vehicle so I dont' know what we're dealing with. This is the most common causative issue for what you're describing but I'm doubtful this is the case here due to how new this car is. If it was a 1995 Subaru in Maine I would almost bet money that's what it is.


1. Can you confirm there are no lights on the dash and haven't been for quite some time?
2. Has this car every been wrecked?
3. Has this car ever seen a major repair?
4. Is there any rust under the vehicle?


This isn't always that easy on asymptomatic vehicles. how can they diagnose something that doesn't exist? They usually replace by guesswork and get it wrong the first...or third...time.
This car has lived in the southwest its entire life and has zero rust. No wrecks that I'm aware of. I'll for sure double check the entire length of all the brake lines. I initially thought there may have been a leak at the front calipers, but the bilateral pattern and viscosity seems like the anti squeal grease on the back of the pads liquified under high heat (no spots near the line, bleeder, or pistons)

Only dash light I know of recently was the airbag light due to the common seatbelt light issue (fixed)

Last major repair was an engine removal for timing chain replacement about 1 year ago.

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