The main purpose of the Black tank Check valve is to STOP ODORS FROM COMING BACK THRU THE LINE. There are 2 types of Black systems. The aftermarket type use the standard City water fill valve that has the built in Check valve and that stops the odor. The 2nd type is OEM and uses the Check valve to prevent the tank odor from going back and out the INLET in the Wash Bay since they INLET does not have a check valve. That is why when you disconnect the hose from an OEM system you have a pint to a quart of fresh water run back out. Doug
