My rear AC quit blowing cold about 7-8 years ago. I called a mobile tech who checked it and said the Freon leaked out. I had him put a port on it. He charged me about $5 for the part and the service call. I put a little pressure in and found the leak. A hole was rubbed through a line. My buddy brazed it up. I pressure checked it again, pulled vacuum and charged. Worked great for a few years. Quit again but couldn’t find a leak this time. I bought a can of stop leak for R-22 and put it in, after pulling vacuum, and recharged again. Knock on wood it has been working since. Been about 5-6 years now.
Not saying this will work for every problem but I spent a little time and a few dollars and so far so good. My front a/c is on the third unit. First one lost Freon about a year after I bought it. Was replaced under warranty. Lasted about a year and it leaked out. I replaced that one so when my rear unit went out, I just figured I would try to fix it and see if it would work.