Sounds like a bad compressor could be the issue. Before replacing it, try using nitrogen to pressure test the system for leaks. Air isn’t great due to moisture. If the compressor isn’t engaging, replacing it might solve the problem, but check for leaks first.

You could get a replacement switch with a lower cut-off pressure if you confirm that’s the issue. But I say is better having a pro take a look.

Since it's holding vacuum well, you might want to stick with R12 if that's what was originally in there. It’s likely to be more compatible with the old system components, especially since the dryer and hoses are original.