Generally large leaks are in the condenser or the evaporator. The condenser sits in front of the radiator and looks like a small radiator. sometimes they take a stone hit that punches a hole. The evaporator is a much harder fix as a lot of the dash area has to be disassembled. A good AC mechanic can add dye and use an ultraviolet light to find the leaks.

Ahhh, thank you Parcon.

It could also be a compressor seal. That was the problem with mine. I found it by charging the system with compressed air and spraying the parts with a solution of dishwashing liquid and water. That and two leaking fitings which simply needed tightening.

If you can determine the basic components like the condensor, evaporator, and compressor are leak free, I would go forward with fixing the original system.

Remember, the prices you are qouting are for the system. It needs to be installed and the original removed.

The biggest problem these old systems have when changing from R12 to R134a is the insufficient condensor size. It is too small to remove the extra heat caused by the higher pressure and therefore warmer in the cabin.

Most compressors fail due to the mixing of the two different oils. Flushing generally gets most out, but there is also always some in the system somewhere. R12 systems used a mineral oil and R134a uses PAG or POE which is a polyester blend. If you use PAG when replacing the refrigerant the oil won't mix and they will coagulate in the system and the compressor will fail. If you use the POE oil it will mix with the left over mineral oil and work fine.

Always change an old receiver/dryer. The filter will fail in the R12 dryer if you use R134a in it.

I quoted the 77 fsm because I have it. There are probably some small differences. I know the vacuum tank was moved down next to the oil filter on the 77 for one. I have not read the 76 manual, but the principle workings will be the same.

The '77 and '76 have a few differences.  Probably 95% of the car is exactly the same, but that leaves 5% that isn't.  It's far better to get the exact year.

 

FAIW, my '78 had a leak in the AC when I bought it.  The problem was the loose low-side flare fitting on the firewall, which was covered with leaked compressor oil.  I tightened the fitting, and the AC has held its charge for years now.  So it can be a very simple thing -- or not.

Edited June 28, 20159 yr by FastWoman