Sometimes to find a leak you have to be inventive. Remove the belt and place a shower cap ( usually the GF or wife's) around the front of the compressor. Came back four of five hours and place your sniffer in there and see if it goes off. Sometimes when you can not find a leak it is the shaft seal. ...

People that keep venting refrigerant into the atmosphere are going to get caught and fined. Refrigerant needs to be recovered not vented.

use Goggle and search MACS

You can have 12volts at the compressor and a defective connection. Use a test light at this connection so there is a load. Most voltmeters do not create a load.

If my failing memory is correct, the POA has an oil bleed line on it. There is a shrader valve in it that is SPECIAL. Do not use a normal replacement.

Early Corvettes were a hard vehicle to keep cool. If I were you, I would keep it original and keep it R12.

So you just vented the extra refrigerant? You're supposed to recover it; not let it get into the atmosphere. Now you do not know how much is in the system.

Check clutch gap. I did not read your entire post.

Recover the charge and weigh the total correct charge back into the system. You may have a marginal TXV at the rear evap. Was it changed?

The rpm at idle is not high enough. Run the engine at 1500 rpm. The high side appears high; it may need a new fan clutch. Run the engine up, mist the condenser with water. If the high side drops, you have air flow problems across condenser. The system may also be low on refrigerant indicating multip...