I believe it's more of a vacuum breaker than a piston check valve. Before the fitting in the UDC broke, if I disconnected the hose, water flowed out. That's why I had a garden hose shut-off valve on it. That was the water in the line on the "potable" side of the anti-siphon valve/vacuum breaker. What I'm referring to as a check valve is like the backflow preventer found in your hot water line from the water heater, or on your city water inlet.
My home sprinkler system has a vacuum breaker/anti-siphon valve on it hanging on the wall outside the house above the valves. When I shut off the water supply in the basement and open the spigot just after that supply valve, I drain the plumbing from there to the vacuum breaker by gravity. Same thing.
With the city water inlet that I replaced the black tank flush fitting with, all I need do is press in the little valve stem for the brass check valve until water stops flowing. The original fitting only had a hollow plastic tube behind it, with no backflow preventer.
Regardless of whether you call it an anti-siphon valve, vacuum breaker, or check valve, I again ask; "How do you propose to force antifreeze through the black tank flusher plumbing and past the (enter name here) valve into the tank?"