Hello John:

I bet you could buy a single o-ring at hydraulics repair shop cheaper than at either a Chevy or Volvo dealer. The trick would be to know the exact size...

The whole rebuild kit for my pump was only around $11 USD from Canada's largest auto parts chain. Virtually all the original o-rings were quite worn-- some that were originally round were very close to half-round, and all were brittle.

If you do just replace the one o-ring on the pressure fitting, be sure to remove the piston (It will probably shoot out when you take the pressure hose off anyway because of the spring behind it) and clean out the screen on the end that points inward. Mine was quite full of black gunk (probably bits of o-ring) which I cleaned out by backflushing with WD-40.

Are you sure it isn't just leaking from the banjo fitting? I think I'd be inclined to get new crush washers or anneal the old ones by heating them up and quenching them in water as others have described here...