How do I replace a toilet flush mechanism?

Flush mechanisms are one of the more satisfying repairs — the parts are cheap, the job is mostly measuring, and a constantly running toilet wastes a genuinely large amount of water.

Safety

What you'll need

• Replacement flush valve or siphon, adjustable type
• Adjustable spanner or large slip-joint pliers
• Sponge, bowl and old towels
• New doughnut washer and cistern bolts if close-coupled

Step by step

1. Work out which type you have.A push-button on the lid usually means a drop valve. A lever on the side usually means a siphon. They are not interchangeable, and buying the wrong type is the most common wasted trip.
2. Isolate the water and empty the cistern.Close the isolating valve on the supply, flush to empty, then sponge out the last of the water. It never empties fully and the remainder ends up on the floor.
3. Measure before buying.Cistern base to the underside of the lid, and the diameter of the outlet hole. Most replacements are height-adjustable, but only within a range.
4. Photograph it before dismantling.Two minutes, and it saves guessing about washer order and orientation on reassembly.
5. Separate a close-coupled cistern if needed.Most drop valves and all siphons need the cistern lifted off the pan. Undo the two cistern bolts and the supply, then lift it straight up with help.
6. Handle the ceramic gently.Never lever against the cistern, and support it while undoing the large back nut. A cracked cistern turns a cheap repair into a new toilet.
7. Undo the large back nut.Underneath the cistern, holding the mechanism from inside so it does not spin. Old plastic nuts can be brittle — steady pressure rather than force.
8. Replace the doughnut washer, always.The thick foam washer between cistern and pan. It is a few pounds, it is the most common source of a slow leak, and it is only accessible while everything is apart.
9. Fit the new mechanism the right way round.The rubber sealing washer goes inside the cistern, the plastic nut outside. Getting this backwards is a guaranteed leak, and it is a common error.
10. Hand tighten, then a quarter turn.These seals work by compression of a rubber washer, not by force. Overtightening distorts the washer and cracks plastic threads.
11. Set the water level and the flush volume.The fill valve sets water level — usually about 20mm below the overflow. Most modern valves also have a dual-flush adjustment. Set both before refitting the lid.
12. Refit and test properly.Turn the water on slowly, let it fill, and watch every joint. Flush several times and check the floor and the back of the pan.
13. Check again the next day.Kitchen roll around the base and under the cistern overnight. Slow weeps show up over hours rather than minutes.

Tips

• Push-button is a drop valve, side lever is usually a siphon. Establish that before going shopping.
• Replace the doughnut washer while you are in there. It is a few pounds and it is the usual leak.
• Rubber washer inside the cistern, plastic nut outside. Backwards is a certain leak.

Common mistakes

• Overtightening the back nut — The seal works by compressing a rubber washer. Excess force distorts it, strips plastic threads and can crack the ceramic.
• Reusing the old doughnut washer — It has taken a permanent set and will not reseal against a disturbed joint. It is the most common cause of a leak that appears the day after the job.

If it doesn't work