Bad seal on black tank gate valve

[epask]

We are new to the forum. 2008 3400RL. For the past several months the black tank will not seal at the gate valve allowing black water in the drain pipe. I purchased a add on valve for the unit that at least allows me to hook up the sewer hose without an unpleasant suprise. After doing some reading I understand that because of the design of the valve and cable this is a common problem. Any suggestions on how to get the valve to close properly?