Hi JGL, I just order the assortment valve kits from ********.com. You can also get them at local parts stores, but they don’t always contain specific Mercedes valves that some models use.

By the way, if all of your schrader valves are leaking, it might be because of the tools you are using. I had a similar problem after I bought a manifold set from Harbor Freight, I love their cheap tools but the quick-couplers kept destroying valves. I learned to install the coupler to the valve with the knob in the closed position, and then open (turn the knob clockwise) just until the pressure gauge begins to register. After that, you can open the knob a little more to check if the gauge climbs any higher; if not you should back it off again. If you screw the knob down all the way, you risk pushing the schrader valve in past its limits. I still use the same manifold sets but I never open the coupler valve all the way.

Another issue I had is with sand or dust in the valve, I use a can of Dust Off to clean and blow out the valves before I attatch any connector to them. If there is contamination in the valve barrel, it can get trapped in the seal and the valve will never close again. I have fixed this by pushing the valve stem in with a tool a few times, to allow the refrigerant pressure to flush out the valve stem - sometimes it works, sometimes it doesn’t. When it doesn’t, I let enough pressure out of the system to be able to replace the valve stem fast enough to finish the job before all of the pressure leaks out and I am forced to vacuum the system again.