I might suggest some alternate ideas.
When I think of the large diameter of the drain and try to think of how the cutout would have to land to actually seal the opening, I have some real questions, but they also involve how the holding tank is treated.
Many people favor using various special paper to avoid clogs but we have never had trouble and use the regular fluffy variety we favor at home, so this would seem to make our black tank waste as bulky and prone to clog as most, if not more.
But when I drain tanks here at home we use a clear plastic insert and a handheld pump to force the waste through a garden hos and to the drain. This lets me get a pretty clear picture of what is draining and it is almost all liquid, not solids! a few gross lumps, etc. but nothing that would clog a four inch pipe unless that flap was pretty well laid flat against the opening!
So I figure the odds of a four inch wide piece of plastic landing squarely on the four inch opening to be way beyond my belief!
I could see that piece being jammed in the opening somewhat more flat like a flap in an air vent and THEN othe rdebris like paper hanging on the flap to build and impede flow.
But that is where how the tank is treated comes into play in my thinking. Is there plenty of water added before solid waste and is there a good chemical to break down that wasted used every trip? We use plenty of waste and regular stock chemicals and do not have solid items like paper or waste which would clog, even if the flap is sideways in the pipe.
A more likely thought for me is that the valve and cables are not working right. One clue is not closing right but then, is there a way to find out if the cable has come loose from the moving plate inside the valve?
I might try this. First get hands on some form of drain gizmo which lets you attach a hose and see the waste inside the gizmo. I would then drain as much liquid out as possible, even if it only trickles out and then let the hose force water into the drain and up to the tank. If water can go in, fill to half full or so and then open the drain and it should rush out.
My thoughts are that if water can go in, it will force any flap laid directly over the hole so that it is moved to a different point and not likely to go back in exactly the wrong spot again!
BUT if the water is slow to go in as well as slow to come out, I'm betting the valve is not working right!
All theory and best of luck to find the problem.
__________________
Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!
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