Why Does My RV Black Tank Leak When Full?

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A leaky blackwater tank is the last thing you want to troubleshoot while on vacation.

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

This quick guide will help troubleshoot why an RV black tank leaks when full so you can get back to enjoying the camper life in no time.

Leaking Black Tank Troubleshooting Diagram

How to check your black tank when it is leaking

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 (i.e., 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 its 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 nearly 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 degraded, 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 a black plastic weld, as long as the gap isn’t larger than 0.125 inches wide.

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.

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, drain pipe connections, and even your black tank itself. Check out our step-by-step RV black water tank cleaning guide to properly clean your tanks.
  • Be proactive and check your connections when you de-winterize your camper trailer.

5 thoughts on “Why Does My RV Black Tank Leak When Full?”

  1. If my black water tank is full and I go to empty it and I take off the cover to attach the hose and it starts leaking what do I do

    • It sounds like you just need to replace the sewer (gate) valve. It’s a simple fix. You can find them for around $20 on Amazon. Though on some models, the valve is located a fair distance from the sewer hose connection, which means if you don’t wait a few minutes after closing the valve before you disconnect the sewer hose and re-install the cap, you may have a small amount of waste water leak out next time you remove the cap.

    • In our trailer the dump.valve and the outlet are quite a distance apart. This would cause sort of a settling effect in the piping. It never really completely drained. Driving would swoosh all that around and when you remove the cap it would leak a bit. We used a separate gate valve from valterra that fits in line with sewer connection on rig and the hose or the cap. This keeps those little remnants trapped in the piping. Not very expensive.

    • First, it depends where the leak is coming from. A pipe connection to the tank or a crack in the tank itself, for example. If it’s a crack in the tank then it depends on the material the tank is made out of. You can use a product like Plasti-Mend if it’s made out of ABS. If it’s polyethylene then you’ll likely need a plastic welder (maybe somebody else can provide another alternative). More information would be useful.


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