That ain't no Hydrovac! It's a simple vacuum-assist canister. It's got a one-way check valve in the feed line. Looks like a vacuum tank on the outside of the frame-rail as well. And it COULD be attached to the clutch.
What was this truck in a former life? Fire trucks, and I'm guessing other construction and work trucks, often used a vacuum assist to pull the truck out of gear from a remote location. For example, on a fire truck that drives the pump through the transmission (usually direct gear), with the drive shaft taken out of line, the vacuum assist allows the pump operator to disengage the pump by turning a knob on the pump panel, thus stopping the pump.
The hydrovac is on the other frame rail. It looks like they may have shared a vacuum line.
-Michael
