One exhaust valve not working

Pull up on the vacuum valve itself and place your finger where the vac hose plugs into it and see if the valve itself will hold vacuum.. Not clear in your description if it HOLDS vacuum..or there IS vacuum.. if it holds the valve in a vacuumed state, then the valve is working.

 

The system has a vacuum tank and vacuum lines to each exh valve via 2 solenoid valves, if one valve is working then try crossing over the vac line from the working to non working valve. That should prove the valve. Also look at buying new solenoid valve as cheap and swap that out.
The solenoid valve is 2 position, open and closed so may be sticking, see if possible to spray some WD40 into its internals to clean it out , if not changing over correctly you may not be getting full vacuum to operate the valve

 

If one valve is working you have good vac on one side, but you dont know if you are getting enough, if sol valve only part way and pulling a little air.

 

I had the same problem on a 99 360. I replaced vacuum solenoid number 178309 and all was well again. Obviously no guarantee that is what your problem is but the parts are cheap and easy to replace and in my case almost 25 years old.

 

Yes, that's what I thought..
You can always try looking for used set of working tailpipes. May be cheaper than a valve replacement.

 

Have a look on Capristo site in Germany to see if they will sell a valve separately, or use a small hacksaw to cut the top off to get to the piston