They say you should but it rarely goes long without pumping out either outside or at our harbors pump out so I have not cut another hole in the bladder and hull for a vent. It works fine and there's no odor.

So I worried that despite the overboard seacock being closed, the vented loop between the WC and tank leaves it entirely up to the jokers to resist the suction. Serious pumps on docks or pump out boats are powerful. But they're pretty tough joker valves in the waste pumps with a metal weight center and thick rubber. My last Y valve weeped a bit after some years, so I went without.

I'd like to change the system to get a larger solid holding tank, but like so many issues like this, it's pretty optimum as is, for how and where we use the boat.

I was boarded off the Cape Cod Canal a couple years ago and this system didn't get more than a quick glance. When the CG saw the overboard seacock closed with no handle(it's on the shelf behind the head ), they were happy.