How would one go about rebuilding one or finding a replacement?

 

What year truck are you working on Chuck?

 

48 f-6. Its under the cab with a I assume vacuum assist

 

Hi. I assume your referring to the hydro-vac. I don't think you will find a rebuild kit or a rebuilt unit of that vintage, it's obsolete. Your best bet, I would think, would be one for a 56, F-600. I was lucky enough to find a cheap core and my regular parts supplier found me a rebuilt unit for $250.00 exchange. I have seen posts on here by people in the U.S. getting the 56 units from Napa.
My 54, M-350 had a hydro-vac from a 52 F-500 on it which I believe would be the same as your 48 F-6. I took the hydro-vac off my 56, C-500 and put it on my M-350. I had to drill two new holes in the 52 mounting bracket and make a new brake line from the master cylinder to the hydro-vac. I then got the rebuilt unit to go back onto the C-500.
I would start my search at Napa but shop around as you may get a better price elsewhere. Someone will be along that is more familiar with U.S. suppliers and re builders than I am. I can only refer to my experience here in Canada.
Hope this helps.