I have a 1983SD and was getting 44 PSIG on the low side compressor running, I am getting 98 PSIG on the high side with gauges that I borrowed with the compressor running. The system is running R-12. Any help on this would be appreciated.

 

the high pressure reading is VERY low if the ac compressor is actually engaged--the suction [low side] could actually be a little high --possibly due to an overcharge--
my guess is the compressor valves are gone therefore cannot build up enough high pressure
can you see any thing in the sight glass?if clear--would let some refrigerant out[recovery machine] until you see some bubbles appear--this would eliminate the "overcharge" problem if it exists-- if the high side is still reading low--[it should be around150-200lbs on a hot day]--i would let an ac mechanic decide before replacing the compressor as it could be something else causing this problem that i'm not familiar with.
make sure belt is not slipping and the clutch of the compressor is actually engaging.
good luck with it!!
barnes