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Old 07-17-2016, 10:03 PM   #1
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Fresh water siphon -almost collapsed tank

Almost collapsed my fresh water tank on my 378.

Today I refilled fresh tank. Coach was tipped quite a bit to drivers side. I overfilled the tank and turned it off when water began pouring out the overflow. Drive back to campsite (200 yards). Water continued to pour out overflow. I figured I had really filled it max.

Water continued. Looked at tank and saw that it had sucked in the sides of the tank! Only way to break the siphon was to get under coach and BLOW air up the overflow tube (getting very wet in the process). Then the tank sucked in air to regain normal size.

Thinking of trying to modify the overflow to provide a vent. Also perhaps to convert the overflow tube so I can gravity feed the fresh tank. There is no way currently to gravity feed tank. We have been using our fresh tank a lot this summer without access to hookups.

Anyone have any thoughts or experiences with this?
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Old 07-17-2016, 10:26 PM   #2
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Is the overfill line the only air vent in that tank? I have had issues with it draining as well but not so bad that it deforms the tank. I wonder if adding an air vent somehow would solve this problem? Seen lots of others with this problem posting but not seen a solution yet.
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Old 07-18-2016, 01:24 AM   #3
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I added a valve at the top of the tank where the overflow tube is attached. I close the valve if traveling in hilly areas with a full tank, where the water may slosh around and start to siphon. I also drilled a small hole in the top of the overflow tube at the highest point to act as a vacuum breaker.
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Old 07-18-2016, 06:28 AM   #4
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Cornelius... great ideas. Can you post pix of your valve? Didn't you have to drill the tank? Would the hole in the overflow alone do the trick? Thanks.
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Old 07-18-2016, 07:23 AM   #5
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I've had the siphon problem a couple of times. One time it emptied the tank. Wasn't happy when I got to the campground with no water. I thought about drilling a small hole in the top od the tube, but afraid it would get water all over if overfilled again.
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Old 07-18-2016, 10:07 AM   #6
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Fresh water siphon -almost collapsed tank

Last winter visiting my parents in Nebraska, I filled fresh tank the night we arrived. The next morning there was an ice skating rink in the street in front of their house. I have installed a 1/4 turn shut off to the vent tube. Just have to make sure you open it before using the pump or the tank will collapse on itself. Didn't have any luck adding extra loops as was suggested on another forum, but don't have clearance at the top of my compartment to do that properly. Saw this 3/4" Radiator flush T repair on the FR3 forum, might try it. Water Tank issue


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Old 07-18-2016, 10:20 PM   #7
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The overflow tube on my 2011 327DS is attached to a fitting that's around 1/4" above the tank. The tube is a 1" tube. This places the top of the tube around 1.25" above the tank. I strongly doubt that any sloshing will throw enough water into the tube for it to come out of a small hole at the top of the tube because the tube is "open" at the lower end and would have to completely fill up with water before water would overflow from the top of the tube.

I'd be more concerned about dust entering the tank through the hole. another possibility is to get a small diameter tube to insert into a small hole in the top of the overflow tube. Attach it to the top of the compartment (adding another 1/2" to the height) then point the open end towards the bottom of the compartment. Now dust needs to travel up the tube to get into the tank.

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Old 07-19-2016, 10:00 AM   #8
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I have not had this happen to me but I only fill my tank 3/4 or less.
I know this happens to some but being it happens usually when over filling the water tank I have a simple solution to stop the siphoning after it starts.
Just look at the fill fitting and remove the screen if you have one.
Inside you will see a plastic pin that is the back flow preventer.



See pin in picture to push in
If you push the pin in it will release the tanks air pressure.
You will need to stand to one side as water may come out for a short period.
Hold the pin in until you get no water and should hear air that will balance the air pressure in the tank.

This is just a simple way to balance the air pressure in the tank.
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Old 07-19-2016, 11:40 AM   #9
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Quote:
Originally Posted by 360true View Post
Cornelius... great ideas. Can you post pix of your valve? Didn't you have to drill the tank? Would the hole in the overflow alone do the trick? Thanks.
The valve I used came from Lowe's, along with the threaded to hose barb fittings and hose clamps. The overflow tube was cut a couple inches after the fitting where it comes out of the tank and the valve was installed into the tube.

To the left of the valve is the small hole drilled in the top of the overflow tube to act as a vacuum breaker for times when the valve is not being used. There were no modifications made to the tank itself, only the overflow tube. Just drilling the hole to create a vacuum breaker may be sufficient. If you decide to try just that, please let us know the results.

If you install a valve, be sure to ALWAYS have it open when the tank is being filled or water is being used in the RV. I only close the valve if I know I will be driving in a hilly area.
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Old 07-19-2016, 12:59 PM   #10
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Quote:
Originally Posted by Cornelius View Post
The valve I used came from Lowe's, along with the threaded to hose barb fittings and hose clamps. The overflow tube was cut a couple inches after the fitting where it comes out of the tank and the valve was installed into the tube.

To the left of the valve is the small hole drilled in the top of the overflow tube to act as a vacuum breaker for times when the valve is not being used. There were no modifications made to the tank itself, only the overflow tube. Just drilling the hole to create a vacuum breaker may be sufficient. If you decide to try just that, please let us know the results.

If you install a valve, be sure to ALWAYS have it open when the tank is being filled or water is being used in the RV. I only close the valve if I know I will be driving in a hilly area.
On my 378 the tank is in behind the black cover. Do you take it off so that you can shut off the the valve or did you cut a hole so you can get at the valve?
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