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Old 11-25-2020, 07:45 AM   #1
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Black/Gray Drain...why no third knife valve?

New 5th wheel owner...went to empty my tanks for the first time. You guessed it.....took the cap off the drain to hook up the hose and was greeted by a foul gush on my arm and worse the smell....seemed to actually burn my throat for about 3 hrs. So it seems my black tank knife valve is leaking ever so slightly. Over time fluid builds up behind the cap. Obviously will have some work on the valve to do.....but my questions....
Why don't they have a third knife valve right before the cap. So in the event of said gray or black tank valve failure you can still take the cap off to empty w/out getting drenched....worse creating an embarrassing natural disaster at the dump station!
Second (odd question but) in cold weather...isn't there a chance of freezing problems in the area between the knife valve and the cap if it's filled with liquid?
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Old 11-25-2020, 08:02 AM   #2
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There are twist on gate valves, some with a clear section of pipe which will allow you to see any "deposits" behind the valve. Just do a search for RV Gate Valve and you'll find options.
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Old 11-25-2020, 08:04 AM   #3
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I have seen a valve that twists on just like the sewer hose does. Probably, you can find it searching the internet.

Yes, it is possible to freeze water in the drain lines. My wet bay has a 120v outlet connected to the inverter. I use a plug in thermostat control module that turns on a 60 watt reflector light at 45 degrees. So far I have not seen freezing temperatures in that bay even with outside temperatures in the teens.
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Old 11-25-2020, 08:08 AM   #4
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You can easily add a third valve :
https://www.ganderoutdoors.com/twist...E&gclsrc=aw.ds

As for your second question about cold weather, it all depends on how the RV is made. If the dump valves are exposed then yes they can freeze. But if is like my coach where the valves are inside a 'wet bay' compartment, a small amount of heat will prevent anything from freezing.
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Old 11-25-2020, 08:11 AM   #5
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Quote:
Originally Posted by PWRmx24 View Post
New 5th wheel owner...went to empty my tanks for the first time. You guessed it.....took the cap off the drain to hook up the hose and was greeted by a foul gush on my arm and worse the smell....seemed to actually burn my throat for about 3 hrs. So it seems my black tank knife valve is leaking ever so slightly. Over time fluid builds up behind the cap. Obviously will have some work on the valve to do.....but my questions....
Why don't they have a third knife valve right before the cap. So in the event of said gray or black tank valve failure you can still take the cap off to empty w/out getting drenched....worse creating an embarrassing natural disaster at the dump station!
Second (odd question but) in cold weather...isn't there a chance of freezing problems in the area between the knife valve and the cap if it's filled with liquid?
Years ago when we had a fifth wheel the valve slowly started to leak. Opening cap released a very small amount of fluid. I added third valve and fixed leaking one at same time. No RV mfg. that I know of has third valve std. assume this is because of the added cost. Pretty easy to do yourself. It only takes one splash like you had to convince me it should be on all rigs if possible. Safe travels

Enjoy the journey
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Old 11-25-2020, 08:28 AM   #6
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Quote:
Originally Posted by Lt Dan View Post
There are twist on gate valves, some with a clear section of pipe which will allow you to see any "deposits" behind the valve. Just do a search for RV Gate Valve and you'll find options.
I have a slight leak on the black valve and the previous owner just left the cap off and let it drain out. Not cool but I do understand why he did that after getting surprised. Not likely, but if you are hooked up to the sewer and the leak is bad enough it could lead to the dreaded pile of poo. I think the PO of our coach had this issue as well.
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Old 11-25-2020, 12:56 PM   #7
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Thanks everybody. I will order the cap with the valve. But must get the knife fixed asap to prevent possible dry black tank scenario. I understand that could be bad.
If black tank get's dry I assume they make some type of compound to try to break up the now dry solid matter?
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Old 11-25-2020, 01:34 PM   #8
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Why don'tcha try this? The plastic ball valve has a screw on cap and the brass elbow swivels. I never get surprised by a leaky valve anymore.
I also use a removable third valve barely visible in the second pic but I use it for back flushing my black tank with water from the grey tank when dumping.
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Old 11-25-2020, 02:08 PM   #9
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Had the same exact thing happen on my coach when I first dumped.

Got rid of the cap...no more surprises.

I tell all my friends when they buy a new RV have the salesman "Demonstrate" how to remove the cap at the dealership.
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Old 11-25-2020, 02:48 PM   #10
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Quote:
Originally Posted by Arch Hoagland View Post
Had the same exact thing happen on my coach when I first dumped.

Got rid of the cap...no more surprises.

I tell all my friends when they buy a new RV have the salesman "Demonstrate" how to remove the cap at the dealership.
Do you have a Biohazard decal on the back of your coach to warn everybody behind you?
https://duramarktechnologies.com/iso...hoC60wQAvD_BwE
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Old 11-25-2020, 02:57 PM   #11
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On past 2 RVs I have ADDED a 3rd/4th dump valve close to end cap
1980 5vr.....3rd valve
2007 5vr.....4th valve (2 grey tanks)

couple of 3" drain flanges, a 3" drain Valve...cut dump line and permanently install
*Twist on is OK except you either take it off to stow which still means 'possible surprise' at end cap OR you leave it ON and then have the added weight hanging out on end of drain line
It would also be too low for my drain line.

Course the ADDED valve seals could also leak...but sure a lot easier to access /repair then the ones hidden up inside the underbelly covering
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Old 11-25-2020, 05:29 PM   #12
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Quote:
Originally Posted by FL420 View Post
Do you have a Biohazard decal on the back of your coach to warn everybody behind you?
https://duramarktechnologies.com/iso...hoC60wQAvD_BwE
Been that way for 15 years and no leaks yet.
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Old 11-25-2020, 05:54 PM   #13
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My motor home has cable operated dump valves. Stupidest setup I've ever seen. The cables are way too long so I took the handles out of the neat looking holders they were in. That let the cables operate more freely but only helped minimally. I use tank valve lubricant religiously to keep the valves operate better. I also spray both ends of the cable with a lubricant (WD40 etc.). The parts man at the dealership said to keep 8 to 10 gallons of water in the black tank at all times. After duping to add it even if you're not going to use it for a while unless in freezing weather. Doing all that has helped tremendously but I still keep a third valve on the quick disconnect.
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Old 11-25-2020, 09:27 PM   #14
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I also added a valve right at the discharge.

Other have recommend using the twist on kind. That would not work in my case as it would hang too low to the ground and I would risk hitting it on something.

I added mine not so much for a leaky valve, but so I could balance my front and rear grey tanks while boondocking.

I cut off my existing twist lock fitting and replaced it with an entire valve. One side was glue on, the other twist lock to fit a standard RV sewer hose. This was cheap and worked out great for us.
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