Hey all, I'm attempting to recharge a 1987 300D with R12 - the car fortunately has not been converted and I do NOT plan on doing so! Anyhow, I got my hands on a manifold gauge set. This gauge is a bit different from others I've seen (it has two extra knobs labelled VAC and REF) and I'm not quite sure what those do. My understanding so far is:

1. Connect the low and high side hoses accordingly
2. Attach the can of R12 to the yellow hose
3. With the engine on and AC controls switched on high, crank open the low pressure knob of the manifold gauge
4. When low pressure approaches ~30psi and high pressure is ~250, AC compressor should engage. Close valve.

To be sure I'm not *over* charging the system, what should the low side pressure be with the compressor off? Does the REF knob in addition to the low pressure knob need to be cranked open in order for R12 to enter the system? Thanks in advance!

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