I recently changed the radiator support in my 73 Fastback and have the front end back together. Does anyone know how much freon the stock system holds?
There should be either a tag on the compressor, or a sticker on the firewall somewhere. If you go to a shop and have it evacuated they will put it on a machine that will put in the correct amount.
You definitely need to evacuate the system for at least 45 minute or so, if you left it open. Actually if you left the system open this whole time, you should just replace the receiver/dryer. The desicant will be saturated.
The stock system was R-12 refrigerant, it will, more than likely, be replaced with either HOTSHOT (an R-12 replacement) made by ICOR, witch accually is more efficient than R-12 and is a direct drop-in replacement or converted to R-134a Refrigerant (compressor oil will need to be changed to synthetic) either way it will have to be charged by pressure not by original system weight as both replacement gases take a different charge of gas to fill the same system as R-12 does.
IMO as an HVAC/R contractor HOTSHOT is the way to go. Any auto AC guy worth his salt can charge your system to the correct charge of either of these gases by pressure.
Hope this helps...