Yes, I've got some a reference to that somewhere. But it depends on which models/year since, depending on year the Eights and Twelves used two different diameter booster cylinders and the 33/34 models had a dash control to vary the amount of boost so the lbs of pull would depend on which valve setting you selected.
You can also calculate it knowing the surface area of the vacuum piston and the vacuum applied.
PS - the BK-13740 suspended cylinder type used on 33/34 Eights and Super Eights provided up to 249 lbs of pull with the brake selector at position #1. (ed-corrected).
If you can find it there is a nice little publication about testing and servicing these units, Servicing B-K Vacuum Power Brakes published by Bendix Products Division, South Bend, IN, 1942.
UPDATE: Doing the rough math, assuming 19 inches of vacuum (probably higher decellerating) and an effective diameter of the booster cylinder of 30 square inches (6-1/4 diam), I get 210 lbs of pull by the booster cylinder. For the Twelve with a 8" diameter booster the same assumptions give about 350 lbs of assist. The Bragg-Kliesrath service manual shows 282 lbs of pull on a 6" cylinder with 20" of vacuum, so by concencus of data and methods, between 210 and 282 lbs of force applied by the booster to the pedal linkage.
