Can rough roads create a suction from the water tank vent?

Pangaea Ron wrote:

I like the idea of a plug or valve in the vent tube while underway. Are there any issues with pressurization while using the pump?

The main fill cap is not air-tight, so it lets in enough when pulling small amounts of water.

I changed the cap retainer so that it attaches to the cap via a nylon bolt through the cap, then double nutted. The bolt spins in the hole, but the double nuts hold the retainer strap. I did this primarily so it would provide and additional bit of an air inlet into the tank while the ventline was plugged yet would not allow much water to slosh out. It seeps a bit, but that's ok.

I had such a problem with this, I found a way to extend both the fill pipe, and the overflow pipe by putting in a hollow tube and then another tube down to a half gallon empty pop bottle for each tube.

In a 1/4 mile drive around the block, the main fill tube had overspilled through sloshing and almost filled the water bottle, and the overfill tube had spilled and filled about 1/4 of the water bottle.

After that, I plugged the fill tube (the main culprit) and extended the overfill tube much higher so it would not spill out of it and still release water when I pumped it out of the tank.

I get some slosh and spill from the vent with full tank. Other than some hard water spots I clean off now and then, it doesn't seem to lose a significant amount of water. It is more of a minor nuisance-a few square ft. of water spots which need to be cleaned occasionally.

In a properly designed and built system the answer is NO.

However... if you fill up the water temp in the tank is generally approaching sixty degrees (normal tap water) and you may be... Well the outside temp is 81 here when I parked (Cooling off as it gets dark) Water expands when heated EXCEPT at the freezing point. (absolutely unique in that "Except" no other material exhibits that expanding when freezing).

Thus the tank, if filled, WILL overflow as it warms.

If I fill my tank all the way to where it comes gushing out the fill opening, yes when I hit the road it sloshes around and comes out the overfill and down the side of the RV.

I make a point of NOT doing this anymore because of the concern of other people/cops when you pull in to fuel up and they see something dripping from the MH and need to point it out to you and ask concerning questions. :R

Thanks all for the responses.

I think that I can visualize water sloshing out, but probably not siphoning... although some of those corners were pretty impressive just south of Yachats. My wife gave me a side glance or two.

I like the idea of a plug or valve in the vent tube while underway. Are there any issues with pressurization while using the pump?

The vent tube is connected to a nipple at the top of the tank - no way it could suction the water out, but it is an outlet for sloshed water. I have a tapered plastic plug I use to plug ours while underway. We still have some loss as the fill is a couple inches higher than the top of the tank, and the fill cap doesn't seal very well.

Could be what you are experiencing on rough & curvy roads too.

I installed a valve to close the over flow after I fill the tank.

I suppose it could. Mine will siphon out a little if I fill it too full.