Why Does My RV Black Tank Leak When Full?

The last thing any RV’er wants to troubleshoot while on vacation is a leaky black tank.

While it isn’t a very common occurrence, it does happen. And to further complicate things, it often only happens when your tank is full.

If you’ve found this happening to you, we threw together this quick guide to help troubleshoot why an RV black tank leaks when full, so you can get back to enjoying the camper life in no time.

If your black tank is only leaking when it’s full, the problem typically isn’t near the drain valve, but rather near the top at one of the connections (vent or drain pipe) or the tank itself has been compromised (ie, a crack near the top).

Let’s look closer at each potential issue.

Cracked Vent Connection

The first place to look is the connection between your vent and your black tank. The seal may have become cracked or the vent pipe could have completely popped out from the tank.

You can check this by trying to twist your vent pipe from the top of your RV to see if it feels loose or by running a hose to the top of your vent pipe and running water down it. If you see the water start to leak under your RV then you’ve found the culprit.

Another giveaway is the presence of odors. When your black water tank is not properly ventilated, due to a crack or the vent popping off the tank, you’ll start to notice a strong sewer smell when your tank gets full.

But, sometimes the crack only starts to separate when the tank is at it’s heaviest from being full (and isn’t securely fastened). So, as a last resort, you may have to physically check the connection itself to see if it’s been compromised.

Cracked Drain Pipe Connection

Similar to your vent connection, your drain pipe, which leads from your RV toilet down to your black water tank, can become compromised at the connection to the holding tank.

Flushing your toilet 10 or so times in a row and inspecting the underneath of your RV may help identify if there’s a small crack in the connection. Be sure your black tank is near full to allow for the weight of the tank to potentially spread the cracked connection open.

Also, like a bad vent connection, the presence of a foul odor when the tank is full can be a giveaway that there’s some sort of connection problem.

But again, the best way to tell is to physically check the connection itself to see if it’s been compromised.

Cracked Black Tank

Once the two obvious weak points have been crossed off the list, it’s time to check the holding tank itself for a leak.

If you have a newer RV it’s very possible that the tank was damaged in the manufacturing process. If you have an older RV, it could just be degradation, as holding tanks become brittle over time.

If you do find a crack in your plastic holding tank you can try sealing the crack with black plastic weld, as long as the gap isn’t larger than 0.125 inches wide.

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Otherwise, you may have to replace your black tank with a new one. You can have your local RV center install a replacement or do it yourself with an aftermarket tank.

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Lastly, going forward, be sure to:

  • Empty your black tank at the proper times. Don’t wait till it gets too full and don’t empty it too often, which is a common mistake. If you empty it at the right time when it’s full, the weight of the contents makes it easier to flush out.
  • Use only the best black tank treatments. Using harsh chemicals in your black tank can harm the vent and drain pipe connections, and even your black tank itself. Check out our guide on how to clean out an RV black water tank for our step-by-step plan to properly clean your tanks.
  • Be proactive and check your connections when you de-winterize your camper trailer.

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