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Many switches are sitting on Schrader valves. As you screw them into place, they depress the Schrader valve, and allow the switch to "see" pressure.

This way you can change the switch, and have minimal loss of Freon and oil, assuming you don't leave the switch loose, and vent off oil and gas. (Should be an o-ring seal that seals up the switch and mounting point/port, so no leaks, even while you are turning the switch, but I've vented some as you move the switch, and turn it before the Schrader valve closes up.)

On my 2001 S8, I ended up adding a short "Extension" to the R134a tank adapter. This fully depresses the Schrader valve on the low side fill port, so you don't have to push on, or wiggle the fill adapter. This extension is perhaps a few mm long, while the insulation is about twice that. You don't need much to do it, but the adapter will not fully open the existing Schrader valve without an extension on there. (Thus why many have had to push on the adapter to get fluid/gas/oil to flow.)

to make one, you need a short piece of insulated solid core wire. (Twisted or braided wire does not work, you want house hold type stuff, solid wire, about 12 or 14ga works fine.)

Just cut a very short snip of wire, and a slightly LONGER part of insulation. The longer insulation you slid down over the Schrader valve, and the insulation holds it in place, while the short solid copper or aluminum wire creates the "extension" that fully opens the shrader valve as the new adapter fitting is screwed into place. This works awesome. No more fiddling with the fill adapters to try and get Freon to flow.

I recommend using oil and Freon fill cans, you always lose oil with a leak, and it's not likely you will over fill the oil, and the system NEEDS the oil to work correctly, and last over time. So, refill with oil and Freon, it's cheap insurance, and the oil actually helps move the Freon, not just lubricates the compressor, so your system will work better too.