What is that blue cylinder looking thing, to the right of the heater temperature control hoses Picture There is some kind of hose attached to the bottom of the blue cylinder heading downwards. The blue cylinder is held in place by a bracket that is mounted on the firewall.
It is the fresh air filter for the Hydrovac, that is a factory location before it got moved to inside the cab I worked on this exact unit in a different truck and removed these parts from wrecks at the salvage yard for some of my trucks. I have some documentation about the air lines and clamps needed to install it in that location. But I don’t know where it is off hand. -s