testing the vacuum system?
 

What is needed to test the vacuum system on a w115 300d. I think I have a leak or the vacuum pump is going out.

You need a Mityvac and a handful of wooden golf tees. The Mityvac will have a vacuum gauge that will help you determine if your vacuum pump is actually producing vacuum. Once that is determined you can check each individual vacuum line and device operated by vacuum by plugging the end of each vacuum line, applying vacuum with the Mityvac and see if it holds vacuum. If it does not, you have found your vacuum leak(s). Do the same with any vacuum operated devices (door lock actuators, engine shut off valve, etc. if so equipped).

There is a check valve that screws into the Vacuum Pump Housing ou can unsrew that and check to see if the inside parts are intact or gunked up.

If the inside parts are gone or damaged you will need to replace the check Valve.

If your Vacuum Pump is a diaphragm type the Diaphragms sometimes need to e replaced.

So I got a vacuum gauge to test if my vacuum pump was working and it is pulling a vacuum. It's pulling 27 in- gh anyone know what it is supposed to pull?

27" of vacuum is great. 15-22" is normal.

Ok so my vacuum pump isn't the issue then guess it's time to check all the other systems.

Excellent vacuum. I usually see ~17" Hg, but recall that is with all tubing connected.

So got my mighty vac and tested all the systems. The only one that wouldn't hold a vacuum was the engine cut off switch.wich I already figured was bad. Having a hard time finding one though. Thought my brake booster was bad but it is holding a vacuum fine so maybe my master cylinder or just needs to be bleed the previous owner said they just did a brake job on it.

Forget the master cylinder because it has nothing to do with vacuum, though there is a flat rubber seal ring between the master cylinder and vacuum brake and that could be leaking vacuum, but that would show up when you do a check on the vacuum booster and you said it holds. If it holds it is not leaking there. Bleeding the brakes or master cylinder will not make and difference either.

I don't know your engine, but easy to find for 1982-85 engines (L-hook on end) and even cheaper & easier for 1986+ engines. Have you tried PeachParts (above)? A few years ago, I bought a lot of 82-85 shutdown valves thru a supplier (made in India) and sold on ebay (much cheaper than most), plus for myself. I was thinking of repeating, but saw them now for similar low price on ebay (relative to M-B, $40).

When you install, smart to add a little clear filter so you can catch it early next time, plus catch the oil before it sucks into the cabin and makes an under-dash mess and drips on your shoe. Use a little lawn-mower filter. Many VW shops have a bin of them.