I have never seen an AC system pressure tested. Instead they are evacuated using a vacuum pump. I bought a cheap one from harbor freight that operates off of a shop air compressor. You such all the air out and make sure it holds the vacuum.
Remember r12 freon is very very expensive, you may want to retrofit to r134a, but the oils used in r12 and r134a are not compatible so while the system is empty you may want to take the opportunity to flush it and get it ready for the r134a.
That is what j do the Red Cheap HF Box.
My buddy who does home A/C has a,pump and does same thing. Sucks the system d own and leaves goes on another call and returns to see if it is ok to fill or not.
