You need someone with a set of gauges to check the pressure and know how to get it charged enough not to trip. I remember the old GM cars would have a fuse under the hood that would blow and keep the compressor clutch from working when they were low. With a 12oz can it was impossible to get it charged enough without the compressor running to keep the fuse from blowing. Our fix was to jumper out the fuse just long enough to fill system. I don't believe I would trust that on my home unit but a repair man might???

Just a few weeks ago I had to have my 40 year AC charged because it was freezing up. It had worked fine all summer but the last hot wave overworked it. It took 2.5 #'s of 438A which works with the existing R22 and old oil. They used pressure and temp on lines to verify a full charge. $150 service call and $100 for 438A. Plus $37.50 to replace schrader valve (tire stem) lol.that was probably the problem from slowly leaking for years.