A great cure for problems associated with low or horizontal mounted radiators. The 200 Series pumps have a built in expansion tank that serves as a fill point and air separator. Returning the pressure cap to the low pressure side of the system allows you to fill the system easily with the pump running and maintains the level by purging air before any water escapes. With a head of water above a self priming pump cavity, this design eliminates air locking and cavitation.