Correct booster for c-10 drums
 

My newly acquired 68 suburban has 4 wheel drum, power brakes. I don't know if this was an option or if someone put a booster on later. So here's the deal - usually when I'm driving slow like in a parking lot, just barely touching the brakes causes them to lock up. I'm talking like its zero or full brake with nothing in the middle. Sometimes it's not as touchy which is weird. On the road it's the exact opposite. I have to push the pedal several inches before I get any real braking action. You can feel that the brakes are there, but you have to dig for them.

Could this be caused by an incorrectly sized booster? A booster going bad somehow? It's really strange how the brakes are night and day like that, and it makes it very unnerving to drive. Can it be a vacuum issue? I feel like the booster just isn't right.

It is quite possible they used a manual brake master with the power booster. Many people with those trucks over the years tend to think they can just put the booster in and have power brakes. However the ratio inside the master is different between a power and manual master. Sounds like what you have going on. I ran a hydroboost on my 68 off of an 88-98 diesel truck. My truck was a turbo LS, so I did this for clearance and so boost did not affect brake pressure (brake boosting was very fun haha)