I have a 98 yukon who's a/c system has given me trouble and more trouble. I had the system charged up around October last year. Ran it a few times then winter came and no need to run it. When it got hot again(in march) I had to charge it back up. It ran for a month and noticed I had to charge it again. Same thing happened so this time I checked it myself and found on the HI-PORT valve I can fill a little air coming from it. Not a great deal, but if I put my finger on it I can tell. Can I just replace the stem or do I have to take it to get the system pumped down and recharged?

Never fool around with any A/C valve, unless the system is evacuated (freon recovered) first. This especially applies to a high pressure port. You could easily loose your sight or worse. After recovery on a CoolTech machine, replace the valve, evacuate (vacuum) the system for 40 minutes to remove moisture and check for leaks, recharge with the proper amount of freon and refrigerant oil (possibly with dye), and check for leaks again. Recovery removes all the freon and most of the refrigerant oil. Five weeks ago I replaced the compressor and accumulator in my Taurus and decided to use BG Universal Frigi-Quiet Refrigerant Oil (Part No. 7018). It comes with a dye. Using a CoolTech machine, the freon and refrigerant oil can be added at the same time.

Get in the habbit of turning your A/C on every couple weeks, even in the winter. It's counter-intuitive, but you can run the heater and A/C at the same time. In fact, it dehumidifies the air and keeps the windows from fogging up.

"UPDATE FROM PREVIOUS"

I had a mechanic check the system. He change the hi pressure valve and stopped working the next day. He checked again the next day and stated the lines may be clogged. He vacuumed the line then took it aloose where the orifice tube goes and freon was pouring out. He changed the tube cause the other was a little twisted and put the lines back together. He proceeded to vacuum the line again then added freon and it was blowing nice and cold on the front and back unit. I drove it home and everything was good. About 12 hours later I get in the vehicle and low and behold it was blowing hot again. I check and the compressor isn't kicking on. If I jump it the compressor kicks on but no change in the a/c temp. Help what can be my problem?

I'm no ac expert ,but it seems like you still have a leak in the system.

Just my thoughts to. There is a rear unit and none of the lines going to it were checked. Also he had stated something about the clutch going out in the compressor. Just trying to get to the bottom of this.