bowats, did this problem just happen one day, or did it occur after the check valve replacement?
There are two check valves on the backside of the water heater, one in & one out. Make sure your winterizing valve is on "Normal", not set at Bypass.
If you do find that the checkvalve(s) is bad, you have several options: 1.) eliminate them totally (depending upon your climaste, the only reason they are in the system is for winterizing really); 2.) replace both of them (or may be possible to dismantle & rebuild the guts); and, the best option IMHO 3.) buy a dual quarter-turn valve Camco winterizing bypass kit and eliminate the pesky check valves once & for all. There's lotsa threads on this; try searching under the Winnie forum for "check valves".