a/c port valve

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. Again the 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?

That valve is replaceable- by itself-- but a good cap with its rubber washer in it will seal off that little leak- by design.. I have to feel you have a bigger leak elsewhere- like at the compressor belly- is your compressor bottom oily/greasy? Is the cradle it sits on puddled with oil? If yours is the OEM compressor, then it would not surprise me if it was leaking....

found no leaks at all. no oil or grease. the compressor was replaced in october when the system was charged up.

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 lil 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?

You have a massive leak--still undefined. Your mechanic needs to find it-- the dye and a good sniffer will find it- Do you have rear air? if so the lines that run back there are in question- look at the grommets for oil....also front evaporator can be a leak source....

yes i do have rear air but he never checked those lines. He did check ever line for the system that is under the hood. He stated that it may be something wrong with the compressor. I don't know but i'll get him to trace the rear system lines. Thanks for the info and if you think of anything else that may help please post.