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Subaru Power Brake Check Valve

Sunday, March 24th, 2013

So I have a new issue with my '99 Subaru Legacy Outback: the check-valve for the power brake booster is sticking when the car sits in very cold weather. Or at least that's what I think is happening.

The point of this valve is to only let air flow one direction between the intake manifold and the booster. The manifold applies vacuum to the booster to "power" it - enabling brake assist. So when the engine is off there is no vacuum present and no power brakes. This is bad in the cases when you're tooling along on the highway and your engine dies. So they put this check-valve in the middle to hold vacuum in the booster should it suddenly disappear on the manifold side. It gives you about 2 pedal pushes worth of brake assist and then you have to stomp on the pedal like in the dark ages.

Recently in the morning and when I leave school at 3:30 I have a couple stops worth of assisted brakes and then the third time I step on the pedal it's very hard to stop the car. This lasts for a few seconds and then the valve breaks free and the pedal drops an inch or so and I have normal brakes again. No big deal, I know how to live with it, but Rachelle would die if she drove my car and encountered this situation.

As far as I can tell the valve is located in the middle of the air hose (highlighted in red below). There is a small bump in the hose similar to a snake that just ate a mouse.

subaru brake boster check valve

I haven't decided what to do about yet. I could replace the hose and valve ($34 from the dealer), but I'm not sure it's defective - it could be a positioning problem. When the car is warm and suddenly cools condensation can form in there and freeze on the valve itself causing it to stick. A new valve wouldn't stop this. I'm wondering if I can reposition the hose slightly so the valve is at the highest point so the condensation runs away from it rather than down onto it. I'll have to play around for a few. Looking at that picture it looks like the valve already is the high point, but I'll have to stick my head in there to see. I'm also racing the weather on this one, once it's above freezing I have a feeling the problem will go away.

Comments (2) Subscribe

#1 - Jan 22, 2014 at 8:10 AM
I got exact same problem just on newer Outback. Have you resolved it?
#2 - Jul 9, 2014 at 7:44 PM
Yeah, I traded the car in for a new Honda Pilot...

Permanently fixing all my Subaru problems at the same time.

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