RV toilet problems: won't hold water, leaks at the base, or won't flush

How a typical RV toilet works

Most RVs use a gravity-flush toilet that sits directly over the black tank. Pressing the pedal or handle does two things: it opens a water valve that rinses the bowl, and it rotates a flush ball (or opens a blade) to drop waste into the tank. A rubber seal around the ball holds a little water in the bowl, which blocks tank odours. The toilet is bolted to a floor flange with a gasket underneath.

Some RVs use macerating or cassette toilets instead. The diagnostic logic below still applies, but follow your manufacturer's instructions for those designs.

Safety: Wear nitrile gloves and eye protection. Empty and flush the black tank before removing a toilet, and ventilate the bathroom. Sewer gas is unpleasant and can be harmful in an enclosed space.

Problem 1: The bowl won't hold water

You flush, water sits in the bowl, then slowly drains away. You may notice odours too.

Causes:

• Debris (paper, hair, mineral scale) caught between the flush ball and the seal.
• A dried, hardened or warped seal. Very common after storage, especially in dry Alberta air, or if antifreeze sat in the bowl for months.
• A worn seal that has simply reached the end of its life.

Fix:

1. Flush with plenty of water while holding the pedal down and wipe around the seal with a gloved hand or soft brush to clear debris.
2. Apply a seal conditioner or lubricant recommended by your toilet's manufacturer. Avoid petroleum products like Vaseline unless the manufacturer permits them, as they can swell some rubber compounds. A suitable rubber seal conditioner works on many models.
3. Leave a little water in the bowl when the RV is parked. The seal stays supple when it's wet.
4. If it still won't hold water, replace the seal. Seal kits are model-specific, so find the brand and model on the toilet (often on the back or under the base) and order the matching seal kit.

Problem 2: Water keeps running into the bowl

If water trickles in when the toilet isn't being flushed, the water valve is leaking internally. You might also hear the water pump cycle on by itself (see RV water pump not working: how to find the problem and fix it).

• Mineral grit can hold the valve open. Try flushing several times firmly.
• A cracked valve is very common after a freeze, because the valve body holds water and is easy to miss when winterizing.
• Most toilets have a replaceable water valve assembly. Turn off the water supply and pump, open a tap to relieve pressure, and follow the toilet's service instructions.

If water drips behind or under the toilet when you flush, check the supply line connection and the vacuum breaker (anti-siphon) on the back of the toilet. Both can crack from freezing.

Problem 3: Leaking at the base or floor

Water or odour around the base usually means the flange seal between the toilet and the floor flange has failed, or the toilet has worked loose.

1. Check the mounting bolts or nuts at the base. They often loosen with road vibration. Snug them evenly, but don't overtighten, because the porcelain or plastic base can crack.
2. If that doesn't stop it, replace the flange seal:
   • Dump and flush the black tank completely.
   • Turn off water and the pump, and open a tap.
   • Disconnect the water supply line.
   • Remove the mounting nuts and lift the toilet straight up. Set it on a towel or garbage bag.
   • Scrape away the old gasket. Inspect the floor around the flange for soft spots, which indicate long-term water damage.
   • Fit the new flange seal, lower the toilet squarely and tighten the nuts evenly.
   • Restore water and test for leaks before calling it done.
3. If the floor feels spongy, the subfloor may be rotting. That's a bigger repair and worth having assessed.

Problem 4: The toilet won't flush or flushes weakly

• No water at all: Is the pump on or city water connected? Is the fresh tank empty? Is the toilet's water supply valve (if fitted) open?
• Weak rinse: Low pump pressure, a partly clogged strainer, or a clogged inlet screen in the toilet valve.
• Ball won't open fully: A dry seal or stiff mechanism. Condition the seal, and check that nothing is jammed in the pedal linkage.
• Waste piling up under the toilet: That's a "pyramid plug" in the black tank. See RV tank sensors reading wrong and black tank clogs: what to do.

Problem 5: Black tank smells inside the RV

Smells usually come from a seal that won't hold water, a dry flange gasket or a blocked roof vent pipe. Check the vent on the roof for wasp nests, debris or a broken cap. Keep water in the bowl, use a tank treatment, and always add a few litres of water to the black tank after dumping so solids don't dry out on the bottom.

Good habits that prevent toilet problems

• Use plenty of water every flush. Water is what moves waste in a gravity system.
• Use paper that breaks down quickly. Never flush wipes, even "flushable" ones.
• Leave the black valve closed until the tank is at least two-thirds full, then dump. Leaving it open at a full-hookup site lets liquids drain away and solids pile up.
• Condition the seal a couple of times a season and before storage.
• When winterizing, make sure the toilet valve gets antifreeze and the vacuum breaker is drained.

When to call a pro

Call a technician if the floor around the toilet is soft or stained, if the black tank itself is cracked or leaking, or if the flange is broken and attached to the tank in a way you can't access. Tank and subfloor repairs are more involved than a seal swap.