The reservoir is just a can that goes around the pump with o-rings sealing the spots that can leak. Once its out and you remove the bolts and big pressure fitting, it just pops apart. Clean, replace o-rings, put back together, paint, and reinstall.
Check to see if its leaking from the center behind the pulley. I think that would be the only place that would make you need to replace it. Of course if you can press the big bushing out and the new one in, you can rebuild it for the price of the kit.
Have a look here:
http://tc.wagoneer.net/sblog/static.php?page=ps_rebuild
I just searched: gm ps pump rebuild
The pulley pulleroffer and presseronner can be borrowed from the parts places.