RV toilet won’t hold water in the bowl

Common symptoms

• The bowl is empty a few minutes or a few hours after you flush.
• The water level drops slowly overnight.
• You smell the black tank in the bathroom.
• The flush ball or blade does not snap fully shut when you let go of the pedal.
• A brand new RV toilet has never held water.

Quick checks before anything else

• Press the pedal halfway to add water, and watch the bottom of the bowl stay closed.
• Let go of the pedal and check that it springs all the way back up.
• Look for toilet paper or grit caught at the edge of the ball or blade.
• Check that the bottom fully closes with no gap you can see.
• Think back: has anyone used bleach, drain cleaner or a scouring powder in this toilet?

Why won’t an RV toilet hold water?

An RV toilet has no water trap like a house toilet. A flush ball (Dometic) or a slide blade (Thetford) closes against a rubber seal at the bottom of the bowl. If water drains past that seal, it goes straight into the black tank, which is why you see no puddle. Dometic says to keep about 1 inch of water in the bowl, because that water is what keeps tank odors out of your RV.

• Dirty seal Paper or hard water minerals stop the ball or blade from sealing. Dometic names mineral buildup as a cause of slow leaks.
• Seal out of shape Common on new RVs. The factory flood test can bend the seal, Dometic says.
• Ball or blade not closing Too much friction, debris in the blade track, or a ball arm knocked loose.
• Worn or damaged seal It still leaks after a good cleaning.
• Chemical damage Bleach, drain openers and harsh cleaners attack the rubber.

Step-by-step: make the bowl hold water again

When your RV toilet is not holding water, clean and reshape the seal before you buy one. These steps follow the Dometic and Thetford manuals. Wear rubber gloves and open a window, since you will be working right above the black tank.

1. Add water and watch it close Press the pedal part way. On the Dometic 300, 310 and 320 and the Thetford Aqua-Magic V, a half press adds water while the bottom stays closed. If the ball moves, Dometic says to let up on the pedal a little. Then release the pedal and watch the ball or blade. It should close completely with no gap.
2. Rinse debris off with the pedal Dometic’s first fix is to slowly press and release the flush pedal several times. The moving water loosens and rinses debris off the ball and seal. Repeat it a few times for stubborn buildup, then add water and see if it holds.
3. Brush the ball and seal Use a soft bristle brush and a non-abrasive toilet cleaner. Dometic names its SeaLand cleaner, and Thetford names its Aqua-Clean. Clean the top of the flush ball and the underside of the seal where it touches the ball. Dometic then says to press lightly down on top of the seal while you brush around its full edge.

   

4. Clear the blade track on a Thetford Thetford’s first fix for a toilet that won’t hold water is to check for and remove any debris from the blade or ball seal track. Hold the pedal down so the blade opens, and wipe out paper or grit with a gloved finger or a soft brush.
5. Free up a sticky ball or blade If the bottom closes slowly or not all the way, the parts may be dragging. Dometic says to clean and lubricate the flush ball and seal with silicone spray or furniture polish. Thetford says to use its Toilet Seal Lubricant and Conditioner or silicone spray, and no other spray lubricant, to avoid damage.
6. Re-seat a loose ball arm on a Dometic Dometic notes that the ball pivot arm can pop out of place while you clean inside the base under the ball. If the ball now sits crooked or won’t close, push the arm back into its pivot point. Do not take the toilet apart to reach it.
7. Recondition the seal shape This is Dometic’s fix for a seal that is out of shape, and the first thing to try on a new toilet. Hold the ball open with the pedal, press lightly around the edge of the seal with a finger so it conforms to the ball, and release the pedal. With the ball closed, pour in 1 quart of hot tap water and let it cool completely. Then check for a leak.
8. Check the seal for wear Thetford says to check the blade or ball seal compression with the mechanism, and to replace the seal if it is worn. Look for nicks, cracks, flattened spots or rubber that feels hard. Dometic says that if cleaning does not fix a leaking seal, it may need replacing.
9. Replace the seal On Dometic 300, 310 and 320 toilets, the flush ball seal changes without tools and without taking the toilet off the floor, with instructions in the kit. Dometic says not to separate the bowl from the base or remove the foot pedal. On a Thetford, order the blade seal for your exact model, and say whether it is the high or low profile.

Dometic vs Thetford: seal differences

Dometic 300, 310 and 320. These close with a flush ball against a nitrile butyl rubber seal. Dometic sells a flush ball seal kit, a water valve kit and a vacuum breaker kit, and the seal and water valve both change with the toilet still bolted down. The model label sits on the toilet base under the water valve. Dometic says chlorine and caustic chemicals, such as laundry bleach and drain openers, damage the seals in toilets and plumbing valves.

Thetford Aqua-Magic V. This lighter plastic toilet closes with a blade or ball seal, depending on the version. Its manual lists a blade seal and a waste ball as separate parts. Thetford says not to use scouring powders, acids or concentrated cleaners, which can damage plastic parts and rubber seals and can void the warranty. Write down the model and code from the product ID label, plus high or low profile, before you order.

Before winter, both makers call for RV (potable) antifreeze only, never the automotive kind. Shurflo’s pump manual adds that potable antifreeze poured into toilets protects their seals, and our RV water pump guide covers the pump side of winterizing. Seals are not universal, even within one brand. For how the whole toilet and tank system works, see our RV toilet problems guide.

When to call an RV tech

• A new seal still leaks, which can mean a cracked bowl or a fault inside the base.
• The ball or blade mechanism is broken, and the fix means taking the base apart. Dometic warns against that.
• Water is on the floor as well, which is a different leak. Our RV toilet leaking guide covers the base, valve and pedal area.
• The RV is new and the toilet never held water. Ask your dealer, since the toilet maker’s warranty may cover it.

Common questions