I have a 1999 GMC 1500 I bought new. It has 163,000 miles on it now and is a great truck. It has the 5.3 motor. My question is should there be a valve in the high side of the air conditioning. I have a leak coming from the plastic cap on the high side. It also does not cool as well as it did over the weekend. Thanks for any help you can give!

 

Either evacuate the system and replace the line, have a good welder cut out and solder in a new valve, put in a new valve core or just get a new cap with a better O-ring that seals down on the leaky valve better.

IMO, best option is to recover, replace hose, vacuum for 90 minutes, charge again and drive on.