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Old 08-25-2016, 08:38 AM   #1
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black tank pressure

Good Morning,
I have a 2016 Bay Star and the last time we went camping, I had an issue with the black tank from the 1/2 bath. When opening the cap for the emptying of the tank there was pressure and some residue sprayed all over me. Any ideas what could be causing this. We were out west and it happened 3 times. Brought it back to the dealer and he filled and emptied twice with no incident.


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Old 08-25-2016, 08:54 AM   #2
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Sounds like you did not fully close the slide valve. Did you turn on the tank rinse to clear it after it finnised dumping? What did the dealer say was the problem?
You should consider buying a clear plastic 45 adaptor to connect to the slide valve before you install the hose. With this adaptor you can see if the discharge from the tank is clear before you close the valve.

To install the adaptor, you can swivel the slide valve up or down to get it in the proper position so you can attach the hose onto the adaptor.


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Old 08-25-2016, 09:22 AM   #3
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How long is the pipe from the shutoff valve to the outlet cap? It sounds like the pressure is building between the two and spraying out when you release the cap. You probably installed the cap in the early mourning when it was cool. The temperature went up and caused pressure to build. If you have room to leave the dump hose attached, you can collapse the hose and put a cap in it. when a little pressure builds, the hose will expand and there will be no increase in pressure.
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Old 08-25-2016, 09:37 AM   #4
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There should be just a short section of pipe between the cap and the blade valve. There are only a few possibilties for any pressure to develope there...

Temperature increase
Drop in ambient static pressure from...
gain of altitude...or dropping barometric pressure
Some waste trapped capable of gas formation...

You'll have to see which is applicable...

If there is pressure in the black tank...it would have to be a blocked vent...
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Old 08-25-2016, 10:41 AM   #5
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I second the idea of a blocked vent stack causing pressure to build up as tank fills ? Worth running a plumbers snake down from the roof if that's where you are vented from.
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Old 08-25-2016, 10:54 AM   #6
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I believe it's the heat build up creating pressure in the drain tube. The tube runs the entire width of the MH, after the valve. No matter how long you let it drain there will be some residual fluid that stays in the tube once you move the MH. That fluid, with the heat expansion causes that little bit of condensation and pressure to build. It happens on ours when it is hot outside. All I do is slightly loosen the cap and you will hear the pressure release then I take the cap off. The pressure is in the tube which is blocked from the tank, not the tank itself. I may get a couple of drips of clear fluid, but it never has sprayed out or caused any mess.
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Old 08-25-2016, 11:56 AM   #7
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As said the vent pipe to tank should release any pressure in tank, check for blockage of vent.
If knife valves are not fully closed water will back up to drain cap, after tank empty check the groove knife blade enters, for q-tip or paper caught in groove slot.
In QT's # 3 there is a BS section with other items you may need to know about, all the links below in signature are for your Newmar.

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