What causes a pyramid plug?
Two habits create it. First, not using enough water — solids need to float to drain, and a dry tank just stacks them under the toilet. Second, and the big one: leaving the black tank valve open while you're parked and hooked up to sewer. The liquid drains away and the solids pile up into a cone right where the toilet drops in.
Once it forms, it blocks the toilet drop and tricks the sensors into reading full. Forcing more down on top only makes it worse.
How do you break up a pyramid plug?
The fix is water and time, not muscle. Close the black valve, then put water back into the tank — flush the toilet repeatedly, or run a hose into it — until the tank is good and full, ideally most of the way. Let it soak for several hours or overnight so the cone softens and breaks apart.
Then open the valve and let the full tank's weight blast it out. If it's stubborn, repeat the fill-soak-dump cycle. A tank-flush wand or a built-in backflush jet aimed down through the toilet helps a lot, and an enzyme or bacteria tank treatment speeds the breakdown.
- Close the valve and fill the tank with water (toilet flush or a hose).
- Add an enzyme/bacteria treatment and let it soak — hours to overnight.
- Open the valve and let the full tank flush it out.
- Repeat if needed; use a flush wand or backflush jet for a tough plug.
- The classic backup: a few bags of ice plus water in the tank, then drive — the ice tumbles and scours the walls.
What if water and soaking don't clear it?
For a truly cemented plug, a flexible tank wand or a soft, RV-safe auger run gently down through the toilet can break the cone — go slow and never force anything rigid that could crack the tank or pipe. If the clog is at the valve or the outlet rather than under the toilet, that's a different fix (often a valve issue), not a pyramid plug.
How to make sure it never comes back
Three rules end pyramid plugs for good. Keep the black valve closed until the tank is at least two-thirds full, then dump all at once so the rush of water carries everything out. Always use plenty of water — far more than feels necessary. And use RV-safe toilet paper plus a tank treatment to keep solids breaking down.
Never leave the black valve open while connected to sewer. That single habit causes more pyramid plugs than everything else combined.
Stuck on this one? Put Major in your pocket.
Tell RV Journey Genie your make, model, and symptom and get a step-by-step fix — by text, voice, or photo — before you ever call the shop.
Frequently asked questions
Why does my RV black tank read full right after I dump it?
Almost always a pyramid plug — a cone of solids and paper built up under the toilet that blocks the drain and fools the sensors. It forms from too little water or from leaving the black valve open while hooked to sewer.
How do I clear a black tank pyramid plug?
Close the valve, fill the tank with water, add an enzyme treatment, and let it soak for several hours or overnight. Then open the valve and let the full tank flush it out. Repeat as needed; a flush wand or ice-and-drive method helps with stubborn plugs.
Should I leave my black tank valve open when hooked up to sewer?
No. Keep the black valve closed until the tank is at least two-thirds full, then dump all at once. Leaving it open lets liquid drain away and solids pile into a pyramid plug under the toilet.
Can I use a drain snake in my RV black tank?
Only a soft, RV-safe wand or auger used gently through the toilet. Never force a rigid snake that could crack the tank or fittings. Water, soaking, and an enzyme treatment clear the vast majority of clogs without one.