5kT A/C line test port thread size?

[Ameer Antar]

Hey, I have a '85 5kT with A/C which uses the earlier GM-type compressor. The system seems to be filled w/ pressure b/c there is freon coming out of the test port (compression fitting, I think) near the upper line going to the condensor. There is a port valve (looks sorta like a tire valve, except for the fitting itself), which is leaking. There is a nylon cap which seems to keep the valve from leaking. I don't have the time/money to mess around w/ the A/C system, so I'd like to just seal the system up until I can mess w/ it. It doesn't seem like the nylon cap would last too long, especially under pressures of daily driving and such. So, I'm wondering if there's a way to fix the internal valve somehow w/o losing all or most of the system pressure. If not, I thought to just put on a new metal cap to replace the nylon one. Does anyone know if this would be possible? Also, how can I tell what size of fitting this is? Where can I get these fittings reasonably? What metal would be most appropriate? I've seen steel, stainless, and brass. I couldn't find aluminum although it seems like the rest of the system is aluminum. I hope someone on the list has had some A/C experience to help me out. TIA.

-Ameer