Thanks Sam!! Michael: I was a car mechanic full time for over twenty years, but only a part time truck mechanic, so what I'm about to say may not apply to all trucks and power steering reservoirs on cars are usually mounted to the pump. The power steering system is not 100% closed. The reservoir is not completely full leaving air room to allow for expansion caused by heat. The caps are usually vented to allow air to escape during expansion. I originally mounted my reservoir without thinking, something that seems to be a habit of mine. Looking back, every reservoir that I've ever seen was mounted higher than the pump. Because the cap is vented the weight of the fluid would probably drain itself to the reservoir if it was lower than the pump and the pump would not be able to pick it back up and even if it did you would probably have constant wines from air in the system. Thanks for the interest!! Thanks for the comments and encouragement!!