The valve under the sink is designed so that if your tank filled up and there was a back flo into the backflush hose, the valve would close and allow the back pressure to drain into the cavity where the valve is located. And if it is in your bathroom, above the garage, it will probably follow the outside of the hose down into the garage. Under normal circumstances, your tank has a pipe venting out the roof which is where most venting should occur. That pipe has a plastic cover over the top. Make sure it is not crushed and that the pipe is not plugged.
When your black tank is full, make sure you pull the exterrior valve for it to start to drain before turning on the backflush water.
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Bill Hutfles
2010 Montana Mountaineer 326 RLT
2012 Dodge Ram 3500 dually, 60 gal tank, Ride Rite Air Springs w/guages & pump
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