GDJT Allroad^2

Since I now have the S4, the wifey drives the A8, which she is thrilled about. We're both not thrilled about the brakes. They were working, but required a lot of pressure to push. The are no leaks....

So we pulled one of the vacuum check valves and found it was bad. A friend I and replaced it with a straight pipe. It seems to do the trick and the brakes are very strong now.

If we can safely run without this check valve, why is it there?

escapA8

Unhelpful comment: If you don't know what it's for it doesn't mean it doesn't serve a good purpose.

ryoung

The check valve prevents air from entering the booster when you lose vacuum, such as during periods of full thottle operation. Even though vacuum is relatively quickly restored when you lift the throttle, the check valve insures that you always have maximum vacuum (and max vacuum assist) available for at least one or two applications of the brake.

Though you will have full braking assist available 99.9% of the time, it's probably a good idea to get a new check valve installed ... to avoid a surprise increase in pedal effort when you don't need one.

BTW: Good job tracking the problem down to the check valve!

ryoung

I forgot or never knew what probably 1/4th or more of the dashboard buttons do, so I'm assuming they serve no useful purpose. I wouldn't feel safe making the same assumption about anything to do with the brakes.

KaMaKaZieP -ALUMIN8-

Where was this check valve located?

KMU

the engine quits.

GDJT Allroad^2

There is a hose from the booster that's more plasticy than rubber that runs forward, through the firewall and cuves towards the passenger side behind the engine block. There is a check valve and then it becomes a rubber vacuum line. It was that check valve that went bad.

KaMaKaZieP -ALUMIN8-

Did you see the other check valve about 6 inches further down the line? Have you checked that one also?