A/C R12 fitting and vacuum.

[DMCJosh85]

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.

[vwdmc16]

You need R12 service fitting. They're like the R134a but they screw onto the threads for R12. Screwing them on the presses a small needle in the center which opens the Schrader valve. You need to screw them on quickly and tightly or else you will lose refrigerant or vacuum in the system another reason why they changed to a quick release Style on R134a. Don't you have any R12 manifold set for Servicing?

[DMCJosh85]

Don't you have any R12 manifold set for Servicing?

The set I have didn't seem to come with anything in mind for this. The manifold screws onto the ports and will work great for allowing the refrigerant to flow into the system, but it came with no such adapter to help with the vacuuming out with the R12 fittings. In my case what I got was the servicing manifold which has the 2 gauges, and 3 outgoing ports for the high, low and yellow lines and of course it came with quick disconnects which will work for the R134 fittings.

Based off of what you told me, should I choose to vac the system with Schrader valves installed on the R12 fittings, I would need this?
https://www.amazon.com/Four-Seasons-...9936162&sr=8-2

[David T]

The hoses on one end are supposed to have a brass piece that depress the valve stem to open the valves. Maybe you just got the hoses backwards on your service manifold? Anyway best practice is to use a valve extractor to withdraw the core and you connect your hose to it. That way you aren't trying to draw a vacuum through such a small valve and can pull a harder vacuum quicker. I use it on the suction (low pressure) side. I also replace the valve cores, they are the biggest source of leaks. I also replace the plastic valve covers with brass ones so they can't leak even if the core does. As for what pressure to fill to, that depends on temperature. You should fill by weight, not pressure.

[TTait]

Exactly. You probably need to unscrew the hoses from the manifold and put them on the other way. The end of the hose that attaches to the car should have a small brass tab at the center of each hose that depresses the Schrader valve when you screw the fitting on to the port. If you don't see the tab, look at the other end.