This isn't on a square body truck, but the next generation GMT 400.... On my '97 C3500 Crew Cab, I had the Hydroboost unit replaced in 2022 and it cost around $1000 at a good shop(a not good shop charged me $600 and they didn't install it correctly or bleed it right so it only lasted two weeks). This was just for the booster replacement and proper bleeding. I replaced the master cylinder two years later when it was going bad; that cost $160 for the part and my neighbor and I did the job including bleeding procedures.
The accumulator can go bad or get loose, and this will give symptoms similar to a bad master cylinder. Also, some replacement boosters need a spacer, but don't always come with one. Pushrod length should be checked as well vs. the new unit, before the core is returned. Sometimes a spacer can be replicated with washers between the booster and master cylinder mounting bolts... this had to be done the last time my truck was in the shop, because apparently the new unit, to replace the one being exchanged under warranty, was just a slight bit shorter and so the brakes would get tight after a few miles of driving, and self apply. The washers fixed it and the truck had great brakes for a week...then some dumba$$ pulled out in front of Mom when she was driving the truck, and now it's probably totalled.....
