Sounds like a low refrigerant level . If its still an R-12 system then the low and high side pressures will be significantly different then if it was an R-134a retrofit. Check with a gauge set. If you low on refrigerant then first find the leak, fix it and do the R-134a update . Common leak areas are the compressor seals and at the O ring fittings. You will need to add leak detector and have an UV light to trace leaks. Repairing the A/C system is much more involved than this so read the service manual and follow the proper procedures..
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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "