Actually, there isn't much need to put antifreeze into the black tank flush line. Any water between the anti-siphon valve and the tank will naturally drain back to the tank. For what may be in the line to the valve, depress the button in the fitting check valve (if there is one) and drain the water out. The original B&B Molders plastic intake broke and from the parts I found, did not have a check valve, just a flow restriction. I replaced it with a brass city water connection w/check valve. I've never pumped AF into the tank flush line and it still works just fine.