You need to have a way to pump the system into a vacuum before the refrigerant goes in. I bought a compressed air-powered "pump" from Harbor Freight Tools many years ago, but it cannot get even close to a perfect vacuum. Now I take my projects to a friend's body shop and use his charging station.
I have the luxury that all my vehicles are Chrysler products, and they all have a sight glass in the receiver drier to show when the system is fully-charged. (That is supposed to be replaced too any time the system is opened for repairs. It has a desiccant in it to absorb a few drops of water). I can just keep adding refrigerant until the vapor bubbles disappear in the sight glass. For all other manufacturers, you have to go by the amount called for on the label under the hood, and program that into the charging station.
You might consider converting to R-134. This is the time to do it. The oil in the compressor has to be changed to a different formulation that is compatible with R-134. The modification used to be real involved and impractical, but it has been simplified to just changing the refrigerant and oil, and adding a pair of port adapters.
A 30-pound cylinder of R-12 used to cost well over $800.00. That was mostly due to taxes, and in most states you can only buy those if you have a license to work on AC systems.
There is a minimum pressure that must be in the system for the compressor to turn on. If that pressure cannot be reached, you'll need to find the low-pressure cutoff switch and temporarily bypass it. That will make the compressor run and draw the low side down so more refrigerant will leave the can. Usually you can get the first can in, but by then there is enough pressure in the low side that no more will flow in from the second can. A trick to overcome that is to place the can in a pot of hot water. Always keep the cans upright so only vapor goes into the system. If you hold it upside-down, liquid will go in. It is being pulled in the low side near the compressor, and they cannot tolerate pumping a liquid. At a minimum, the compressor will lock up for a little while. More commonly the valves will be damaged.
Friday, March 3rd, 2017 AT 5:49 PM