[ScottinSoCal]

Got back from San Diego yesterday, and we did take the convertible. But while down there it started doing something strange with the brakes. Need some feedback.

Pulling up to a stop, the brakes start out fine. As the pedal goes down, the power boost gradually drops off, until there's none left. It doesn't lope, as if there's a vacuum leak, but there's no brake boost. Pumping the brakes does nothing, and it does it every time I brake. I've seen brake boosters go bad before, but they didn't act like this. It didn't get better or worse during the trip. I'd lean toward a vacuum leak, but there's no sign of it in the engine. I don't want to just start throwing parts at it - I can't afford that.

I've never had a brake booster check valve go bad - do they feel like this?

 

[isthisdave]

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Although I've never seen it, I suppose a check valve could fail in such a way as to restrict flow enough to allow one depression of the pedal to be "powered", but then need higher rpms and time to again create enough vacuum for another cycle.

When you say "as the pedal goes down", has the pedal travel changed?

 

[ScottinSoCal]

When you say "as the pedal goes down", has the pedal travel changed?

No, the travel feels the same, it's just that power boost fades out as the pedal is depressed. There's no recharge time needed, either. I can press the brakes twice, quickly, and get exactly the same response. It starts out great, and then loses boost as the pedal gets closer to the point where the car will stop. It's like the vacuum bleeds off proportionally with the pedal going down. There's no hiss of leaking air, though, and no engine stumble as if there was a major vacuum leak. I've never seen anything like it.

 

[isthisdave]

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The no obvious signs of a vacuum leak is the odd part. There is only one moving part though, so.......

 

[rowdie]

I have something like that in the ''''84 500SL. New booster and master cylinder has not cured it completely. It has ABS and I think it needs a good bleeding with a power bleeder.

Won't help you now but I plan on checking the valve by trying to apply vacuum to what I know is a good check valve to see if it holds vacuum. Too damn cold right now. Maybe someone in a warmer climate can do it.