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Ask Robert Your Own Question
Robert, RV Mechanic
Category: RV
Satisfied Customers: 13283
Experience:  18 yrs. RV experience including Cummins North West, Country Coach, Monaco Coach - vendor certified.
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When I thought my black water tank was completely empty, I

Customer Question

When I thought my black water tank was completely empty, I pull forward and the drain fill with black water again. Had new valves installed in the past year. What is causing this?
JA: How many miles are on the car? Have you noticed any leaks lately?
Customer: None before this. About 7200 miles on Montana 3400RL 5th Wheel.
JA: What year is your Montana?
Customer: 2011
JA: Are you fixing your Montana yourself? What have you tried so far?
Customer: Have it at the repair shop but they are puzzled by it too.
JA: Anything else you want the mechanic to know before I connect you?
Customer: Had this trailer 5 years and it didn't do this until the last trip.
Submitted: 10 months ago.
Category: RV
Expert:  Robert replied 10 months ago.

Hi my name is Robert....

So you are saying that you drained the black tank and then it was full again? IS the black tank filling when you are sitting hooked up to city water etc...


Customer: replied 10 months ago.
No. When it sits the drain does not fill. When it moves, it does.
Customer: replied 10 months ago.
Mechanic is saying that water is moving from the black water tank into the shower gray water tank???
Customer: replied 10 months ago.
When first noticed, I also saw water dripping off the outside of the gas pipes that run alongside the Montana so water was also coming from a point above that horizontal gas line. I have not hit anything with this trailer or underneath it that would cause a leak in the tank.
Expert:  Robert replied 10 months ago.


I would be looking for another grey tank valve some units will have two grey tank valves, one for the main grey water and a secondary for the galley drain, that valve if you had it is normally located above the rear wheels. Really no way the black and grey water should be mixing at all unless it is getting by the valve. The other issue would be if there was a vacuum breaker not working that is normally plumbed to the tank or toilet. They should not be tied together with the exception of the discharge valves.... Robert

Customer: replied 10 months ago.
This unit has three dump valves. One black water. One gray water (shower) located next to the black water. A third one which is for gray water for the kitchen and it is located between the passenger side trailer wheels on the frame.
Customer: replied 10 months ago.
The shower gray tank valve was replaced one year ago. It should be fine.
Expert:  Robert replied 10 months ago.


Right understand about the three valves. They should all be shut when you are traveling, if the water is not getting through the valves I suspect there could be a vacuum breaker at some point that is not working the vacuum breaker prevents the tanks from mixing. Not sure what else this is going to be as the tanks should be independent of each other. Robert

Sorry can not do a call at this moment, another RV expert may take the call....

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