In my experience here in St. Louis, you will have a hard time (a very hard time) finding a "kit" to rebuild the hydrovac yourself. My guess is that the kits are all snatched up by the rebuilders. Fortunately, the GM hydrovacs can be rebuilt, as mentioned, quite routinely, by dealers. There should be a tag on your hydrovac, similar to a carburetor tag, with a number that a rebuilder uses to identify the hydrovac. Ask at NAPA, or check with your local "big rig" service center. They will no-doubt have a resource for this, and given the number, should be able to quote you a price for a rebuilt unit.
It's usually a core-swap deal, so you give your old one in, and they send a newly rebuilt one back. By-the-way, be sure to take all the fittings, such as elbows, flare adaptors, etc. off the hydrovac before sending it in. The rebuilt ones don't usually include those.
Hope this helps,
-Michael
