The schraeder valve is the common name for a tire core valve. The valve in the low side fitting looks just like a tire valve, but the rubber is different on it.
Just push in the tip with a screwdriver like letting the air out of a tire to see if there's anything in it.

There is an O ring at each hose joint on the AC system. If dried out or leaking they need replaced, not hard, but you do need to get a can of R12 mineral oil to lubricate them before installing ( coating with the r134a oil will dry them out ) If you see what looks like an oil stain around an ac hose fitting, that oring is leaking.

Volvos are actually about the easiest thing on the roadto maintain, especially a 940. Most of what you need is here in the 700/900 faqs. The haynes manual is handy to have . For anything real specific like AC system or wiring diagram, go to www.volvotechinfo.com and you can buy the factory green manual for the specific system
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually