Assembly is so quick and obvious often time people skip reading the instructions. We cover this in our videos, but here it is again. The point worth noting:
The insertion of the tubes into the bottom of the controller must go through the drum lid (correct side up) BEFORE tightening drum lid to the controller.
The three different diameter tubes will only connect in the correct hole. Fool-proof as long as you first insert them through the holes in the drum lid (lid being correct side up) and are certain the tubes are FULLLY SEATED into the controller, PRIOR to tightening the lid to the controller. Proper seating may take extra effort especially with the largest tube, no worries, you can not over-push the tubes into the controller.
The entire point is to insure trouble free connections, the tubes must be seated BEFORE tightening the drum lid. Tightening the lid first (before inserting and seating tubes) could result in an incomplete seating of the tubes. This would result in not being able to draw fluid properly into the pump and not being able to build pressure.