Some years back I bought one at a Car-Quest store from a knowledgeable parts man with gray hair just like mine only more of it. The part number that was available was for a 2-ton so that's what I ordered. It was a reman from A-1 Cardone. I gave them a core from an old Cadillac and nobody complained. This was maybe 20 years ago and the price was $175 if memory serves. It has worked ever since in my 1-ton. Hydro vacs were an RPO (regular production option) on 3800 and 4400 trucks and standard on 2-tons. The frame was even drilled for the bracket from the Cadillac. I fabbed the vacuum lines and a big vacuum tank and ran the both the brakes and wipers off the vacuum tank till the wiper motor gave out. That was a setup I'd seen on trucks pulling trailers with vacuum brakes.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.