Anti-freeze coming out the hot water heater

We went on a short trip to Penn. this week, so the Landmark got left on a storage lot in Colorado, and with the sub-freezing temps expected we had to winterize it. I flipped both bypass levers and pulled the drain for the HWH. As I was running around inside turning on faucets, my father was changing the anti-freeze bottles. He said that about a gallon of antifreeze ran out of the HWH even though it was bypassed. Is my check valve shot for the HWH output, or is there something else I must do to prevent backpressure from filling the HWH?

On my unit, there is a line running vertical, you cant see the valve, because the valve is on the back side of the line. It bypasses the water heater. That valve has to be turned.I dont know why the valve isnt facing you as you look at the lines?
Ron Ps just was out dewinterizing and the valve is in a" vertical "line with a tee that is turned so you cant see it !!