I am really hoping someone has the answer(s) to this one.  
Beginning about a week ago, my brake pedal goes to the floor slowly at stops and then the Vanagon has little braking and starts rolling.
The first thing I thought of was "vacuum check valve". I looked for it and was gone, having been replaced with a short length of plastic tube. I am surprised by this, because I don't remember removing and not replacing the check valve. I know there is supposed to be a check valve. So who knows for how long the check valve has been missing.
So I replaced the tube with a proper check valve, but the pedal still won't hold, and goes to the floor.  
The questions are: Can driving missing a check valve  ruin the vacuum brake booster?  
Or, can a bad master cylinder be the problem?
Are the symptoms of a bad master brake cylinder as described?

I have had almost every other problem with a car you can think of, but never a bad master cylinder.
The one that is in the Vanagon is not the original, having been one of many swapped out until we learned the real reason for the mushy brake pedal after replacing the rear shoes: we didn't adjust the new shoes til they touched the drums, then back them off.  

So, the master cylinder is one that was sitting in a garage for an undetermined amount of time, exposed to dust and humidity no doubt.  
Before I possibly go in the wrong direction, and remove the master cylinder for the fourth time, does anyone know how I can be certain the master cylinder is bad? Please be as technical as you want.  

Robert
1982 Westfalia

Robert  

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