Walter, we are having trouble understanding why when mounting the pump vertically, the motor needs to be on the bottom with the head on top? We have one mounted this way and another mounted with the motor on top and both are priming and seeming to work properly. I am certainly not implying you're wrong, just looking for more explanation. Could you further clarify how or why air could get trapped if the motor is on top? If it's working fine for us with the motor on top, is it okay to continue to use it that way or is this causing some kind of damage we can't see? We read your tutorial, which was awesome by the way, and everything makes sense except this part about mounting the pumps vertically.