I thought a check valve only keeps water from travelling in the opposite direction than intended or against the normal water flow. For instance, a pump sending water to the tank above has water moving up. A check valve in that line would keep the water from coming back down into the sump if power was to go off.
Not sure you need a check valve from what you're describing. Sounds like you got something similar to a leacky faucet that won't stop dripping. Still, i've never used a doser, so maybe I'm missing something.
"Life may not be the party we hoped for but while we are here we might as well dance!"