How do I find a leak in the AC system...

[bandit0067]

Guys, I got a one owner 71 GMC 1500 with factory air. The original compressor was not locked up but the clutch hub would not engage. I rebuilt the original engine awhile back and just now trying to get the ac going. I put in a remanufactured compressor and found I have a leak. The factory installed system has not been opened and I ran the new compressor a short bit I can find no evidence of leaks in any outside joints of oil. Looked for leaks with a soap solution on the pressurized system showed nothing. Been a while since I did some trouble shooting on one of these and was wondering if any new technology is out. I see cans of drop in with leak stop and leak check but not sure they would be ok in my original 50 year old system. Any ideas or opinions on how to proceed.....thanks.....