[CarolinaHD]

1967 and prior will have a a single master/booster on the firewall with 1 line feeding all 4 wheels.

68 to ~1980 will have 2 master cylinders, 1 on the firewall and 1 under the floor like sweetk30 said. Each master feeds 2 wheels.

Sometime in the early 80s, they started using the Delco-Moraine hydroboost. It bolts to the firewall like a traditional hydroboost and has dual outlets on the master cylinder.

According to the Rock Auto catalog, 1983 was the first year for disc brakes.

Most if not all c50 have juice brakes.

Most but not all c60 have air brakes

The 4 bolt pattern on my 1967 c50 firewall was the same as the bolt pattern on the hydroboost off a 2007 c5500 and a 2003 3500. It's safe to assume that any gm hydroboost will bolt to another firewall/truck from the 60s till at least 2011.