Check valves are used in both the liquid and suction lines to prevent the backflow of refrigerant.  RParts Magni-Chek check valves are especially chosen for their extremely low back pressure (i.e. cracking pressure).  This prevents the forward-flow pressure drop which can be a problem with lower quality valves.  In addition, the valves also include leak proof magnetic checking action.  What does this signify?  It means that there is no spring or ball bearing to wear out for a longer lasting check valve. 

The Magni-Chek check valves are copper bodied and can easily be soldered.  They can be mounted in any position but be sure to note the flow direction before mounting.