That sounds like a great truck, welcome to the gang! My 52 3 ton Chevy fire truck, Canadian built, has the sm420, and the vacuum two speed. She also revs pretty good, closer to 50mph, but it's got a 78 292 power plant.
To the point though, since mine does use the main drive shaft to pump in 4th gear, the drum style with mechanical inner and outer shoes parking brake is located on the rear of the pumps gearbox. This way it holds the truck's rear drive shaft thus the wheels, steady while pumping.
