As stated it's the joker valve. A manual pump
uses raw water
. I'm going to guess your raw water
is saltwater. The salt
mixed with uric acid breaks down joker valves fairly quickly. Expect to replace them every few years.
The discharge line fills at the top of the tank. This means that fluid (blackwater) sits in the discharge line between the joker valve and the tank fitting. Without the joker valve, the contents can leak back into the bowl.