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Old 05-21-2012, 08:34 AM   #1
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Holding tank suction when removing cap

I have searched this site without an answer so maybe someone can help. After driving my mh to a destination there is a strong suction created when removing the cap to my holding tanks. I always empty my tanks before travling but there is usually some residual "water" underneath the cap as I remove it. There does not seem to be a leak when sitting still. This just occurs after traveling. Any ideas?
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Old 05-21-2012, 08:53 AM   #2
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Sounds like a clogged vent pipe to me, or the tank is full enough to block the vent. You should never have suction in the holding tanks.
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Old 05-21-2012, 08:56 AM   #3
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So I will take a shot at this and see how close I am.. First it would be helpful for you to post Make, Model & Year of your MH in your signature..
So the sucking pressure is when you unscrew the cap and before opining the valves?? It sounds like a venting problem. Maybe insects nests in the roof vent, mud or dirt. It could be building up pressure and causing the valve to pull air back into the tank..
Now we will wait for the "old pros" to come on and give the answers.
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Old 05-21-2012, 08:59 AM   #4
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Ok...More information. This is a 2011 Newmar Class A. My tanks are never full. I empty them before I travel all the time. I did look at the vent tube and it does not appear to be clogged. I do not get any foul odors and it only happens after traveling.
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Old 05-21-2012, 09:04 AM   #5
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Traveling where? Low altitude to high altitude would result in negative pressure in a SEALED container, as would traveling from a hot climate to a cold one. Regardless, I can't think of any time holding tanks should have any pressure (positive or negative), because they are supposed to have an open vent to the outside.
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Old 05-21-2012, 09:10 AM   #6
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Have you tried opening the toilet flush gate before opening the cap to see if you hear any suction.
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Old 05-21-2012, 10:08 AM   #7
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If the vaccume is just between the cap and the dump valve this space is not vented so vaccume or pressure will be there if a temp change, or baro pressure changes so this is normal.
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Old 05-21-2012, 10:15 AM   #8
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If the dump valve is closed and not leaking and the suction is between the valve and the cap, it must be simple tempurature changes causing this issue. You might try slightly opening your dump valve (tank empty of course) to see if that dumps the suction.
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Old 05-21-2012, 04:15 PM   #9
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Suction when you remove the caps. I assume the dump valves are closed at this time.

You can get a cap with a hose fitting on it, this is often used to drain gray water via a smaller hose.. This cap you can "Crack" the hose cap to break the vacuum, makes it easier to remove the bigger 3" cap.
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Old 05-21-2012, 08:20 PM   #10
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Quote:
Originally Posted by jazzguy View Post
I have searched this site without an answer so maybe someone can help. After driving my mh to a destination there is a strong suction created when removing the cap to my holding tanks. I always empty my tanks before travling but there is usually some residual "water" underneath the cap as I remove it. There does not seem to be a leak when sitting still. This just occurs after traveling. Any ideas?
Mine did this one day last trip out. Caused me to raise an eyebrow too! I do as you, generally emptying prior to hitting the road. Mine also does the "residual" couple table spoons liquid as well - sometimes. Anyway, I wrote it off as the black tank is/was running under a vacuum going down the road (just that day, all the time, dunno). As my slide valve O-rings have a leak - that black water tank vacuum is being transfered to that area between the valve and the cap - creating quite the suction trying to remove it.
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Old 05-21-2012, 09:43 PM   #11
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If your vent is in a position in the air stream that generates low preasure it could draw air out of the tanks. Empty tanks offer a better vacume than full tanks. This could draw air around the valve seals out of the cavity between the valves and the cap. The negative preasure in this cavity could draw some fluid back into the area above cap when you stop. The valves are designed to hold water back under normal preasure but if your vent draws a significant vacuumed when on the road this is what is probably happening. The little fluid inthe bottom of the tank is sucking bubbles up through the valve and then a small amount of fluid is being drawn back down when you stop.
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Old 05-22-2012, 05:59 PM   #12
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Being a 2011, it must be one of them thar new fangled "green" diesels. More than likely, the the EPA has gotten together with TSB and other Government agency's and probably have your engine intake hooked up to your black warter tnk to burn all that stuff in thar that you left, to get it out of the air we breeth. I like my oldun and it dont stunk.
Try leaving the valve open and the cap off and leter suk.
Sorry folks, just a bit of humor, hope you enjoy and the previous posts are the ones you should listen too.
I never travel with an empty tank, as I am sloshing and cleaning as I go down the road. Unless I am going into the "unknown" and then I leave about a quarter tank to slosh and clean. I always dump on arrival given I have a place to dump. I never travel empty and gauges always work.
Rob
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