A series of questions for the community regarding some A/C work I hope to do soon. The question being: Since the Schrader valves (AKA Valve Stems) on the R12 fittings will not open when pulling vacuum as its pulls against the closing direction of their allowed 1 way of flow, how can one pull vacuum on the car with these installed?

For the sake of the pressure test I removed them for checking for leaks over several hours, but obviously I'm going to have to put the Schrader valves back in and pull vacuum again when I'm ready to charge the system so Im uncertain how to get past this. Searching around online I find plenty of R134 quick disconnect adapters but without going that route how can I do what I want to do? I cannot seem to find a plain adapter for my manifold gauge set that will allow a straight vac line to plug into the R12 and keep the Schroeder valve open for vacuum suction.

Additionally as I understand it the car should take 12-14 Ounces of RedTek, I get that I should have a scale on the cans as I inject it onto the system, but I wanted to ask if there was a low pressure PSI I should charge up to that would be considered to be a fully serviced refrigerant system.

Much appreciate any help pointing me in the right direction.