Dometic 310 RV toilet water won't stay in the bowl: what do I replace?

Answered by RV Parts Giant · Upland, CA · October 7, 2026

Short answer: The flush valve. On the Dometic 310 the flush valve is the seal-and-valve assembly that both controls the flush and holds water in the bowl, and a worn one lets the bowl drain into the tank or leak water in. It is a small part and a short job, not a new toilet. Harsh bleach and non-RV chemicals harden the seal and bring the problem back.

A bowl that empties itself between uses lets tank odour into the bathroom, and a bowl that slowly fills on its own wastes fresh water. Both symptoms on a Dometic 310 come from the same assembly, and both are fixed without lifting the toilet off the floor.

What is failing

The 310 is a foot-pedal gravity toilet, and the flush valve is the seal-and-valve assembly that controls the flush and holds the water in the bowl afterwards. When its seal hardens, pits or collects grit, the bowl no longer seals and either drains or weeps. Owners see the same part behind a weak flush that will not clear the bowl.

Confirm before you order

  • Bowl drains dry over a few hours. Flush valve seal. Replace it.
  • Water keeps trickling into the bowl with the pedal untouched. Also the flush valve assembly on this toilet, since it controls the flush water as well.
  • No water enters the bowl at all. Check the water supply and the pump before blaming the toilet.

The replacement flush valve we stock is the genuine assembly for the standard Black Series RV toilet, which is the Dometic 310 Series in white fitted to Black Series HQ and MDC campers. If your 310 came in a Forest River, Keystone, Coachmen, Jayco or Thor, send us the model label from under the seat and we will confirm the fit before you pay.

How to change it

  1. Shut off the water supply and press the pedal fully to empty the bowl. Keep a towel and a small container handy for the water left in the valve.
  2. Note exactly how the old assembly seats before pulling it. The orientation matters when the new one goes in.
  3. Fit the new assembly, then run several flushes. The bowl should hold water and the area around the seal should stay dry.

Keeping the new seal alive

Keep the seal clean, use holding-tank chemicals rated for RV seals, and keep harsh bleach out of the bowl; it hardens rubber and the bowl starts draining again.

When it really is the toilet

If the ceramic-look PU bowl is cracked, or the pedal mechanism is broken rather than the seal, the whole Dometic 310 is a direct swap on the standard two-bolt floor flange, and at half a pint per flush it is also the right choice for anyone trying to stretch a small black tank. Both the valve and the toilet ship from Upland, California.

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