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Siphon out the drain hole


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I added 440 sacs on top of the rear hard tanks, via rerouting the vent line. The factory pump seemed to fill the bag fine, draining worked fine, just left an inch or tow of water in the bag like I expected. But I did have a problem filling. This is what I think happened. When filling I think the pressure in the factory hard tank was enough to not only push water out the vent line into the sac but also the drain line. Once the water flowed through the drain line a siphon was created which didn't stop. I made sure that the bag was still below the drain line and it was, so gravity wasn't the cause.

This is my solution while on the water. I stuck my finger in the drain hole on the hull until the bag was full. Now I had to figure out how to break the siphon. I quickly replaced my finger with my mouth and a lung full of air and blew hard until i heard air bubbling in the hard tank. And it worked. The siphon was broke and we had some awesome surfing.

So now the question is; does anyone now how to prevent the flow of water out the drain hose while filling? I would like to stay away from inserting a ball-valve on the drain line.

Here are some pics of the install



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Someone mentioned a duck valve. They are supposedly used on AC condensers. I looked at them and it seemed it would prevent a siphon but still allow one way flow under pressure from the drain pump. Just need to see if I can find one with a 3/4" ID.

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I have one of these on the fill lines already. It was factory installed with the MLS.

Will it also work on the drain side and prevent the siphon from being created? Because the pressure in the hard tank is pushing water out the vent line(which I want in order to fill the sacs) but also pushing water out the drain line, which the creates the siphon. If under pressure will the vented loop still work?

It just occurred to me that I may have two problems

1-while filling pressure is pushing water out the drain line

2-once fill pumps are turned off a siphon exists causing draining.

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  • 1 month later...

Has anyone just tried replacing the stock vent line with a larger one (maybe 1" or 1.25")? You would have to reduce it at the thru-hull connection to 3/4" and get a 3/4" NPT to 1" barb connection for the fat sac side, though. But with this setup, the siphon may not hold because of the larger tube diameter. Just a thought.

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  • 3 weeks later...

My solution has been to put a wine cork in the drain hole. It was worked flawlessly. Just need to remember to pull it out when you go to drain.

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My solution has been to put a wine cork in the drain hole. It was worked flawlessly. Just need to remember to pull it out when you go to drain.

Hey we did that too - worked for a whole season then we installed a ball valve inline.

The inline valve is really the ez way to go and only take 2 seconds to deploy or ploy.

and since its inline it can be placed in some very accessible places

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Would a check valve in the vent line work?

I too have added a 750 on top of the hard tanks except I plumbed them with new 1250 pumps (one for fill and one for empty) and but a tee on the vent line for the new sac.

sometime its easier just to leave the pump running but I would like to have it consistent.

What do you think?

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