Repair when Refilling

Testing The Toilet Tank Water For A Leak Into The Toilet Bowl

Leak Detector Tablets or food coloring can be used in a tank to confirm a leak but these tablets do not verify where the leak is occurring. The best troubleshooting procedure for determining the leak point is using this water test.


  1. With a full tank, turn the water off at the shut of valve. DO NOT flush the toilet.
  2. Make a mark in the tank at the current water level.
  3. Wait 30 minutes. If the water level has dropped make a new mark at the new water level and continue to wait.
  4. Repeat step 3 until the water level no longer changes.


The water level will drop down to the level of the leak and stop.

  1. Water drains to the lip of the flapper –

A) The flapper may be worn.

B) The drain seat that the flapper rests on is damaged.

Recommended Solution A:  Change flapper

Recommended Solution B:  Repair flush valve drain

Please watch this video to see how to repair the flush valve drain:


2. The water drains to the bottom of the tank – This happens when the gasket that seals the flush valve into the tank is deteriorated and worn.

Recommended Solution: Change flush valve

  • If a leak occurs at a flush valve gasket in new installation, the lock nut securing the flush valve to the underside maybe leaking. Fluidmaster instructs that the flush valve lock nut should be tightened no more than 1/2 turn beyond hand tight.

3. The water drains down 1/4 to 1/2 inch from your mark and stops.

A) The refill tubing may be positioned incorrectly.

B) The overflow pipe may have a crack.

Solution A: Re-position refill tube so it does not enter into the overflow pipe – if possible attach tubing to a clip to hold refill tube above pipe opening.

Solution B: Change flush valve