Hydro-Vac. Mostly found on 1 1/2 ton and larger GM trucks, standard equpment on many 2 ton and larger after '55. It was an option on the 1/2-1 tons starting around 1955. It's more or less a second master cylinder with a booster on it. I've seen them mounted in various locations, usually on the frame under the cab on the passenger side rail. They can be found in salvage yards, but overhaul parts are difficult, if not impossible, to get. That's why I passed on one I could have got for $5, couldn't get parts.
A modern equivalent would be the Midland Booster. Jeg's and Summit both carry them. those are commonly used on Pro-Mod and Pro Street cars, and mount bqasically the same.
