it was made to relieve pressure in those engines,
the design of it was to pull vapors out of the motor while it was going down the road. at a certain speed a slight vacuum is produced in the pipe due to its location and design. fresh air is introduced into the top of the engine on the vents on the valve cover or a oil fill breather and the air along with any combustion gases were pulled out via the draft tube. the draft tube does'nt work if the truck isnt moving and only starts to work at a certain speed.
