The flush valve is the larger plastic outlet at the bottom of the tank — the seat that the flapper sits on. It's the part that the water rushes through during flush. Replacement is less common than flapper replacement; typical service life is 12–20 years. You replace the entire flush valve when the flush valve seat itself has degraded (mineral buildup that won't clean off, hairline cracks, or pitting that prevents a flapper from sealing). For TOTO, the engineering differs by model line — here's the map.
TOTO flush valve types by model
| TOTO line | Flush valve type | Diameter | Replacement complexity |
|---|---|---|---|
| Drake (original, CST744SL) | Standard gravity flush valve | 2 inch | Easy — universal Korky 2032BP fits |
| Drake II (CST454CEFG) | Double Cyclone flush valve | 3 inch | Moderate — 3" universal flapper fits but seat is TOTO-specific |
| UltraMax II | Tornado Flush valve | 3 inch | Moderate — TOTO-specific seat geometry |
| Vespin II | Double Cyclone | 3 inch | Moderate |
| Aquia IV | Dual-flush canister | 3 inch outlet | Hard — TOTO-proprietary dual-flush canister |
| Eco Drake / Eco UltraMax | Reduced-flow gravity valve | 2 inch | Easy — universal 2" fits |
| Neorest series | Tankless siphonic — no valve | N/A | N/A (service through TOTO dealer) |
When to replace the flush valve (vs just the flapper)
The flapper is the rubber seal that lifts during flush. The flush valve is the larger plastic seat the flapper sits on. Most people only need to replace the flapper. You need to replace the full flush valve when:
- The flush valve seat (the plastic surface under the flapper) has visible mineral deposits that don't clean off with vinegar
- The plastic flush valve seat is cracked or chipped
- The overflow tube (the vertical tube extending from the flush valve to ~1 inch below the tank top) is cracked or broken
- You've replaced the flapper 2-3 times in 2-3 years and the toilet still phantom-flushes
Replacement procedure for Drake-series (2" or 3" gravity flush)
- Shut off water, flush, sponge tank dry.
- Disconnect the supply line at the bottom of the tank.
- Unbolt the tank from the bowl. Two brass bolts inside the tank, under the rubber sponge pads. Hold the bolt head from inside with a screwdriver while loosening the nut from below with a wrench.
- Lift the tank off the bowl. Set it on a towel on the floor.
- From outside the tank, remove the large jam-nut securing the flush valve assembly (channel-lock pliers — sometimes plastic-shielded to avoid damaging the porcelain).
- Pull the old flush valve out from inside the tank.
- Install the new flush valve assembly. For Drake II: TOTO THU135S or Korky 2032BP. For original Drake: Korky 2003BP or universal 2".
- Replace the gasket and tank-to-bowl bolts ($12 Korky 481BP kit). Always replace these at the same time — old gaskets degrade alongside flush valves.
- Reseat the tank on the bowl. Tighten the bolts gradually and evenly.
- Reconnect supply line. Turn water on. Test flush, watch for leaks at all three sealing surfaces (supply connection, tank bolts, tank-to-bowl gasket).
Replacement for Aquia IV (dual-flush canister)
The Aquia IV's dual-flush mechanism is a vertical canister inside the tank — similar to Kohler's AquaPiston but TOTO-engineered for dual-flush actuation. Replacement is:
- Disconnect the dual-flush actuator linkage from the top of the existing canister.
- Lift the canister straight up out of the tank — it slides free without tools (no jam-nut). Canister snaps in via O-rings on the flush valve seat below.
- Inspect the seat (the bowl-side flange the canister sat on) for mineral buildup; clean if needed.
- Insert the new canister (TOTO TSU99A.X dual-flush kit, $45). Verify the dual-flush settings match the toilet's original spec.
- Reattach the dual-flush actuator.
- Restore water and test flush. Verify both the half-flush and full-flush volumes are correct.
Cost summary
- Drake / Drake II flush valve assembly (Korky universal): $20–$35
- TOTO OEM flush valve (Drake II): $35–$55
- Aquia IV dual-flush canister (TOTO OEM): $45
- Tank-to-bowl gasket + bolts: $12–$25
- DIY time: 60–90 minutes (gravity flush) or 30 minutes (canister)
- Plumber service call: $220–$380