Leaking Head Discharge vented loop valve

[Roni]

On my 1989 Island Packet 35, the Forespar vented loop valve (duck bill style) leaks liquid when pumping out my Jabsco Twistnlock head. Enough to create a 6-8" wet spot on a paper towel laid across the valve. The particular vented loop is between my head and the holding tank. The vent also makes a squealing noise when pumping (air escaping?). I've removed the valve and cleaned it without success. I then replaced it and still have the same issue. Additionaly, I get a small amount of back flow into the bowl (head has not been rebuilt in several years).

Something is causing elevated pressure in the line. While the dirt daubers love to clog my holding tank vent line, I still get the problem after insuring the tank vent line is clear and opening the deck pump out cap to relieve any potential tank pressure. I am able to pump my tank without issues. The only other item between the head and holding tank is a Whale diverter valve, which allows over board discharge when in an approved area.

Thoughts? I'm thinking a restriction of some sort since the bowl contents will make it to the head. Any issue plugging the old cap with epoxy to seal the vent while docked? (short term fix)?

[Tynesider]

Hi Roni

Had a simular problem with my 'Twist & Lock' on my Jabsco toilet but I do not have a holding tank.

Spent days messing around including like you changing the vented loop valves,removing pipework to check they are clear etc, in the end give up and just purchased a new toilet pump assembly (currently about £60 here in UK), fitted it in 1/2 hr and no more problem.

I can only 'think' the rubber flap in the pump 'may' have been sticking at times when I flicked from flush to pump out, but for £60 it's not worth messing with kit parts as they can be very fiddley to assemble.