Even with a manifold gauge set, it is a bit difficult to get a perfect charge. Outside temperature and humidity dramatically skew the readings. Adding 'a bit of freon' simply does not cut it these days. To easy to overcharge. A few years ago, I emptied the system, evacuated to a vacuum, and installed the closest approximation of the exact required charge that I could. I do not see any other way to do it right. The days of R-12 and a site glass for charging are in the past.
The gauge set and a pump are cheap enough and if you do the job right, the compressor will have the best chance of lasting.
Personally, I hate the newer freon. It stresses the compressor and in general, does not cool like R-12. So, when we had 98 humid degrees a few days ago, I was very happy that the GOAT's AC was able to keep up.
Last edited by littlerbigfoot; 07-23-2019 at 09:54 AM.