The pre-treatment cover to my Jet system does not have a riser and the cover is buried.

The first thing to do is locate the inlet of the tank.  If you have an exposed outlet riser, as most plants do, the inlet location should be the opposite direction that the outlet riser is from the aerator riser.  Next, measure towards the inlet from the center of the aerator riser approximately sixty inches.  This location should indicate the center of the pretreatment riser.