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Receiver-drier is on the high side of the system. There is nothing to do there in adding charge to the system.
The service ports on a '90 are on the rear face of the compressor. (Assuming you have the OE Diesel Kiki compressor.) I have found easiest access is from beneath (you do have ramps, don't you?). One is labeled "S" and one is labeled "D". High side is "D" and low side is "S". Charge into the low side (suction side) with compressor running.
When connecting to the fitting, first thread the hose's fitting on loosely (not enough to depress the valve core). Then open the valve at the can tap or gage set and wait until refrigerant comes out at the service port where you loosely threaded the fitting. Then tighten the fitting and close the valve. This makes sure there is only refrigerant from the can to the interior of the compressor. Air trapped in the system creates strange problems.
After connecting, start engine, engage compressor clutch, open valve, charge till no bubbles in sight glass. With R-12 the system should be relatively insensitive to small overcharges, and the total charge amount is fairly large on this vehicle, so I would go ahead and charge in increments of an entire can. You may need to set the lower portion of the can in some warm (not hot) water to raise its pressure slightly and ensure flow from the can to the system. Also you may need to raise the idle speed by finagling the throttle cable somehow (this will lower suction pressure). Keep the can upright so as not to charge liquid into the suciton side.
When done, close valve, shut down car, crawl under, screw the fitting off as quickly as possible (there will be some loss) and put the cap back on.
Good luck. It's not as difficult as is seems, but does require careful lab technique.
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