1988 GMC 1500 pickup A C unit. Where would the(Filler), high side be on the AC lines..
you want to fill the high side? look for the hose that goes into the cab (the low side hose) and along it you will find the fitting > this might help you...when you find it than the other one is the high side.
Kelly's picture is correct BUT it's the low side (as shown in the picture) that you want for charging. NEVER hook up to the high side for charging! HTH. -Jim
BTW, unless the system has been converted to R134a you have R12 in there which can't be purchased anymore in the United States. -Jim
Looking for a Used Sierra 1500 in your area?
CarGurus has 58,734 nationwide Sierra 1500 listings starting at $1,599.
Search GMC Sierra 1500 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
GMC Sierra 1500 Experts