Running an open system without pressure and losing coolant?

Well slip this in the 5/16" overflow tube and voilà; you have mild pressure to keep the fluid in your system. 

In an non pressure system the fluid evacuates due to:

• excess coolant
  
• thermal expansion 
• excessive flow from higher speeds 
• blocked and clogged tubes 
• water pump impeller design flaws

This handy dandy little check valve will not suck back but it is a great tool for keeping the fluid in for longer.