I can tell you what I had to do on my truck, and it only had 8,021 miles on it in 2012 when I got it, I had to rebuild the entire Brake & Clutch Hydraulic system including the Hydra-Boost, I had all but one wheel cylinder leaking, and the master cylinder was full of dirt, and this truck spent most of its live inside, from 1965 to 2004 when it was taken out of service.
I covered most of this in my Builds & Journals theard,
1965 GMC 3500 Rescue Squad, starting on page 4.
Hope this helps you out some, keep in mind my truck is a tad smaller, its a 3500.