RV Toilet Not Holding Water in the Bowl

You fill the bowl, walk away, and come back to a dry bowl and a whiff of black tank. Or the water drains in an hour, or overnight. Either way the seal that is supposed to hold a couple of inches of water above the tank is not sealing.

In order of likelihood: debris or mineral scale between the seal and the flush ball or blade, a dry seal that has lost its flexibility, a seal that is actually worn out, and then the uncommon ones, a warped blade or ball, a seal reinstalled wrong, or a pedal linkage that does not let the valve close all the way. Most of the time the fix is a gloved finger and a plastic scraper, not a parts order.

You need gloves, something plastic and thin to scrape with, a soft toothbrush, vinegar, and 15 minutes. If you end up replacing the seal, add an hour and a socket for the closet bolts.

How the bowl holds water

An RV gravity toilet is a bowl sitting directly over the black tank with a valve between them. Press the pedal halfway and the water valve admits fresh water. Press it fully and the valve at the bottom of the bowl opens and everything drops. Release it and a spring closes the valve.

What the valve looks like depends on the toilet:

• Dometic 300, 310, 320 use a plastic flush ball, a hemisphere that rotates open and closed. A rubber ball seal sits in a groove in the base, and the ball presses up against it when closed. Water in the bowl sits on top of the seal and ball.
• Thetford Aqua-Magic V, Style II, Style Plus use a flat blade (slide valve) that slides across the opening. A rubber blade seal with a thin lip rides on the blade surface. When the blade is closed, the lip rests on it and holds the water.

In both cases the seal is a rubber ring pressed against a plastic surface. Anything between them, a grain of sand, a flake of scale, a bit of paper, holds the seal off the surface and makes a path for water. Rubber that has dried out does not conform to the surface and leaks around the smallest imperfection. The bowl water is also what keeps tank odor out of the rig.

Step 1Clean the seal and the groove before you buy anything

Put on gloves. Flush the bowl to rinse it, then hold the pedal down so the ball or blade is open.

On a Dometic, reach into the bowl and run a fingertip around the underside of the seal lip, where it meets the ball. You will feel a ridge of grit or crusted scale. Work a plastic scraper or an old credit card gently around the groove between the seal and the base to lift the debris out. Do not use anything metal; a scratch in the ball becomes a permanent leak path.

On a Thetford, with the blade open, you can see the seal lip from above. Wipe the top of the blade and the seal lip with a rag, then run the scraper along the seal groove on the bowl side of the blade. Close and open the blade a few times to flush loose material through.

Let the ball or blade close and look at the surface it seals against. White or tan crust is mineral scale from hard water. Soak a rag in white vinegar and lay it over the ball or blade and the seal for 30 minutes, then scrub with the soft toothbrush and rinse. Repeat once if the scale is heavy. Vinegar is safe on the seal rubber and the plastic; harsher descalers may not be.

Refill the bowl and check it after an hour. Most bowls hold at this point.

Step 2Lubricate the seal, and what to use

A seal that holds for an hour but drains overnight after cleaning is dry. The rubber has stiffened and does not follow the ball or blade surface.

Use a silicone-based toilet seal lubricant. Dometic and Thetford both sell one under their own names, and plain food-grade silicone grease or silicone spray does the same thing. Work it into the seal lip with a gloved finger, cycle the valve ten times, and refill.

The forum debate, settled by the manufacturers: petroleum products degrade some rubber compounds. That rules out Vaseline and petroleum-based plumber’s grease for regular use, and Thetford’s manuals say so outright. Olive oil is a common habit; it does not attack rubber, but it goes rancid and washes away in days. Silicone is cheap and lasts months.

Step 3Replace the seal

If a cleaned, lubricated seal still will not hold water for a few hours, or the seal is visibly cracked, flattened, or glossy and hard, replace it.

Part numbers. Dometic 385311658 is the seal and gasket kit for the 300, 310, and 320 series; it includes the ball seal and the bowl-to-base gasket. Thetford 34117 is the blade seal for the Aqua-Magic V. The Style II and Style Plus use different seals. Check the model sticker on the back of the base and verify the part number against the Thetford parts diagram before ordering; the Aqua-Magic names have been reused across several generations.

Dometic 300/310/320 ball seal replacement:

1. Turn off the water to the toilet (the shut-off behind it, or the pump) and flush to empty the bowl. Disconnect the water line at the toilet’s water valve; have a towel ready.
2. Remove the two plastic caps at the base and unbolt the two closet bolts (7/16 or 1/2 in nuts). Lift the toilet straight up and set it on cardboard. Stuff a rag in the flange opening.
3. Separate the bowl from the base. On the 310 and 320 this is two or more screws or clamps around the seam between the ceramic or plastic bowl and the base. Lift the bowl off.
4. The ball seal sits in a groove in the base around the flush ball. Pull it out. Clean the groove completely: scale, old lubricant, grit. This is where most “it still leaks after the new seal” problems start.
5. Clean the ball itself with vinegar and inspect it. A ball with a crack, a flat spot, or a scratch deep enough to catch a fingernail needs replacing (Dometic sells the ball and shaft as a kit; verify for your model).
6. Lubricate the new seal lightly with silicone, press it into the groove with the lip facing the right direction (it only sits flat one way), and install the new bowl-to-base gasket from the kit.
7. Reassemble the bowl to the base, set the toilet back on a new floor flange seal, and bolt it down evenly. Snug, not cranked; the base is plastic and cracks.

Thetford Aqua-Magic V blade seal replacement is done from above, with the toilet in place on most units:

1. Flush to empty and hold the blade open.
2. Reach into the bowl, grip the seal, and pull it up and out of its groove. It is a stretch fit.
3. Clean the groove and the blade with vinegar and the toothbrush. Dry it.
4. Press the new seal into the groove, working around the circle so the lip lies flat on the blade. Lubricate the lip with silicone.
5. Cycle the blade ten times and refill.

If the Thetford seal will not seat, the toilet comes off the floor and the bowl separates from the base the same way the Dometic does, and the seal installs from below.

Step 4Why it still leaks after a new seal

A new seal that leaks on day one is almost always one of these.

The groove was not clean. A grain of scale under the seal tilts it. Pull the seal, clean again, reinstall.

The seal is in backward or not fully seated. The lip has to face up toward the bowl on a Thetford and sit flat in its groove on a Dometic. Press around the full circle with a thumb and feel for any spot that sits proud.

The ball or blade is scratched or warped. Hold a flashlight at a low angle across the surface. Scratches from a metal scraper or a dropped tool, or a blade warped from a hard freeze, will not seal against any rubber. Replace the ball or the blade.

The valve is not closing fully. On a Dometic, the pedal returns on a spring and the ball should rotate until it stops against the seal. If someone adjusted the cable or the spring is weak, the ball stops a few degrees short. Look at the ball from above with the pedal released; it should be centered under the seal. On the 310 the cable has an adjustment nut under the base. On a Thetford, the blade should fully cover the opening with the pedal up; a stretched return spring or debris in the blade track stops it short.

Bowl-to-base gasket, not the seal. If the Dometic bowl separates from the base, there is a second gasket between them. If it was reused or pinched, water weeps out between bowl and base. This shows as water on the floor around the toilet base, not a dropping water line.

Step 5Water on the floor is a different leak

If the water line in the bowl holds but there is water around the base of the toilet, stop looking at the ball seal.

• Floor flange seal. The rubber or wax-free seal between the toilet base and the floor flange. It compresses once; reuse it after pulling the toilet and it leaks. Dometic 385311462 is the floor flange seal for most Dometic gravity toilets; Thetford has its own for each series. Verify, and buy one whenever you pull the toilet.
• Water valve or supply line. A weeping water valve or a loose compression fitting on the supply line drips down the back of the base. Dry everything, flush a few times, and feel behind the toilet.
• Vacuum breaker. On Dometic 300-series toilets the vacuum breaker behind the bowl can crack and spray or drip when the water valve opens. It is a cheap part.

A floor flange leak puts black water on the subfloor every time you flush. Treat it as urgent.

Step 6Keep the seal from drying out

Rubber seals fail from sitting dry more than from use. A rig in storage for three months with an empty bowl comes out with a stiff seal that may or may not recover.

The common practice is to leave about a half inch of water in the bowl with a drop of dish soap during storage. The soap lowers the surface tension so the water wets the seal lip instead of beading, and a thin film of soapy water keeps the rubber supple. Top it up when you check on the rig. In freezing storage, use RV antifreeze in the bowl instead; a frozen bowl cracks the base or warps the blade.

Lubricate with silicone at the start of each season and after any deep clean, and the seal lasts years.

What counts as holding water

A toilet seal is not a perfect seal. A bowl that holds its level overnight and loses a half inch over a week in storage is behaving normally; a little evaporation and a little seep are built into a rubber lip on a plastic ball. A bowl that drops visibly in an hour, or is dry by morning, needs attention. Do not chase a quarter inch a week.

Symptom → likely cause

Parts and tools mentioned

• Dometic 385311658 ball seal and gasket kit (300/310/320; verify against your model sticker)
• Thetford 34117 Aqua-Magic V blade seal; Style II and Style Plus use different seals (verify)
• Dometic 385311462 floor flange seal, or the Thetford floor seal for your series (verify)
• Silicone-based toilet seal lubricant (Dometic or Thetford branded, or food-grade silicone grease)
• White vinegar, soft toothbrush, plastic scraper
• Nitrile gloves, 7/16 or 1/2 in socket, Phillips screwdriver

Questions people ask next

Can I use Vaseline on the seal in a pinch? Once, to get through a trip, it is unlikely to ruin the seal. As a habit, no; petroleum softens and swells some rubber compounds, and both manufacturers say not to. Silicone grease costs a few dollars and is sold at any hardware store.

The bowl holds water but the toilet smells. Is that the seal? If the water line holds, the seal is doing its job. Smell with water in the bowl is usually the tank vent (a blocked roof vent cap, or wind pushing tank air down the vent), a floor flange seal that lets gas past, or a dirty tank. Start with the vent cap on the roof.

How often does the seal need replacing? With silicone lubrication and water kept in the bowl, five to ten years is common. Replaced every year is a sign the seal is being damaged by something: a scratched ball, harsh chemicals in the bowl, or petroleum lubricant.

Does a bowl cleaner hurt the seal? Bleach-based cleaners and anything with pine oil or solvents dry out the rubber over time. Use a cleaner labeled for RV toilets, or vinegar and a brush.