Ed, It's not that hard. Go on Ebay and get the r12 fill hose as the clamping is different on an r12 system then the r134. Then take Seafoam up on his offer of R12 cans. [I myself have about 12 cans that I bought a few years back when R12 was cheap. I even bought an R12 gauge set].
What you will have to do first on your '90 is disconnect the battery for a couple of minutes to clear the codes. Then hook up the hose [with the can attached but the valve off]. Then start the car, make sure the temp is set at lowest cold and highest fan setting and open up the valve and hold the can upside down until it is empty. You probably only need one can. But if Seafoam has a couple extra take him up on that.
As was mentioned earlier probably the front seal on your compressor has started to leak. I have been told that is common on GM cars.