I just bought a '86 GMC step side with a rebuilt motor. The owner wasn't sure what motor it was, but myself and everyone that has looked at it believes it's a 350. It's been rebuilt with an Edelbrock manifold and 1405 carb. It also has a high performance camshaft but no idea which one. I plan to pull the motor and have someone go through it but for now I'm trying to get the truck in good working order. Here is my issue.
My power brakes do not work unless the engine is above 1,500 rpms. I installed a Jegs vacuum reserve canister to help with this but it made absolutely zero difference. The only two things connected to the vacuum port on the manifold are they TH350 tranny and the brake booster. I disconnected the tranny, put a plug over that hole and tested the vacuum lines and connectors for the brake booster. All checks out. I show roughly 4" of vacuum at idle and roughly 15" at 2000 rpms. I'm being told that is likely due to the camshaft. I'm considering a vacuum pump kit to compensate but thought I would seek comments first.

 

most oem motors produce 18 -20 inches of vacuum at idle. your engine must have lumpy idle with that valve overlap. another option is a hydro-boost system that the factory uses on diesels and 3/4ton trucks. The vac pump is probably the easiest/cheapest fix.

 

Sounds like a cam with a lot of over lap. You should have 15-20 inches of mercury vacuum. Valve timing could have something to do with it also. Also there should be a check valve in the vacuum line to the booster. Make sure the one way AKA check valve is working. And again there should be a one way valve on your canister so when the vacuum from the motor goes away... theoretically at high RPM you still have booster vacuum. If all else fails a pump maybe needed but that is pretty extreme it must have some crazy *** cam in it.