I'll need to do a little checking to see if the GMC master cylinder bore and stroke matches up to anything Chevy used. I haven't had a chance to inspect the truck I bought "sight unseen" except for some pictures yet, but I'm pretty sure there should be a Hydrovac somewhere in the mix, and the PO says it definitely will need a new or rebuilt MC and some other brake work.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!