fj12ryder wrote:

On some models/brands certain configurations allow the low point drain to act as a siphon, which could certainly be annoying. Sounds like yours was just draining, like it supposed to. The overflow will not have a valve unless you add one, like some have done with the siphoning effect. But you can cause the tank to explode if you don't open the valve when filling the tank. Not a good thing, and why some people are hesitant to add the valve.

DallasSteve wrote:

I have a Jayco Jay Flight. I filled the fresh water tank, but now water is draining out from the Low Point Drain under the trailer. Is there a way to close that drain or is it supposed to remain open?
Edit: Never mind. It stopped draining water and the Fresh tank reads full. I guess it's just an overflow for when it gets too full.

valhalla360 wrote:

The low point drain shouldn't be open but many trailers have an overflow hose that will allow excess water out if you try to overflow the tank.

It may be worth checking how full the tank really is. It's not uncommon for the overflow connection to be well below full. We had a trailer where the 40gal fresh water tank would syphon off almost 10gal if it started overflowing. Not a big deal if on a full hook up site or boondocking for a night but if you are off water for multiple days it's a nuisance.

Added a simple valve to the end of the overflow hose. Typically left it open so it would still perform it's purpose but if boondocking, I would close it once it started to overflow (and obviously stop filling the tank). This allowed full use of the tank when boondocking for multiple days.