Kohler Toilet Flush Valve

"Kohler flush valve" is a search that splits into two completely different products. If your Kohler was built before ~2007 or you have a Cimarron, Highline, or Wellworth, you have a Class Five flush valve — a standard 2-inch flush valve at the bottom of the tank with a rubber flapper sitting on top. If your Kohler is a Memoirs Stately, Maxton, Adair, or Persuade (or any model labeled "Kohler 1.28 GPF AquaPiston"), you have an AquaPiston canister — a vertical white cylinder that lifts during flush. The two repair paths are entirely different.

How to identify which flush valve you have

Lift the tank lid and look at the bottom of the tank:

• Class Five (traditional): You see a circular rubber flap (the flapper) hanging on a chain attached to the flush handle's trip lever. When you flush, the flapper lifts and water rushes out around it through the flush valve below.
• AquaPiston (canister): You see a vertical white plastic cylinder approximately 6 inches tall. The trip lever doesn't have a flapper chain — it pulls a small lift mechanism on top of the cylinder. When you flush, the entire cylinder lifts upward.

The two valves are not interchangeable. A Class Five tank cannot accept an AquaPiston canister without bowl-flange modification (and vice versa).

Class Five flush valve — replacement path

The Class Five flush valve has two main wear components: the flapper (the rubber seal that lifts and falls; covered separately in our Kohler flapper page) and the flush valve seat (the larger plastic outlet underneath that the flapper seals against). The flapper fails far more often than the seat.

When to replace the flush valve seat (not just the flapper)

• You've replaced the flapper twice and the toilet still leaks slowly
• The flush valve seat has visible mineral buildup that doesn't clean off with vinegar
• The plastic flush valve seat is cracked or pitted
• The flush valve overflow pipe is broken or loose

The Class Five replacement procedure

1. Shut off water at the wall (1/4 turn shutoff valve)
2. Flush to empty the tank; sponge out remaining water
3. Disconnect the water supply line at the bottom of the tank
4. Unbolt the tank from the bowl (two brass bolts inside the tank, usually under foam pads)
5. Lift the tank off the bowl carefully
6. Remove the large jam-nut securing the flush valve from outside the tank bottom (channel-lock pliers; sometimes plastic-shielded)
7. Pull the old flush valve out from inside the tank
8. Insert the new flush valve assembly (Kohler GP1059291 or Korky 2032BP universal — both fit)
9. Replace the gasket and the tank-to-bowl bolts ($15 kit; always replace these at the same time)
10. Reseat the tank on the bowl, tighten gradually and evenly
11. Reconnect the water supply and turn on. Check for leaks at the supply line, the tank bolts, and the bowl base.

Cost: $25–$45 part. Plumber labor: $180–$280. DIY time: 60 minutes for someone moderately handy.

AquaPiston canister — replacement path

The AquaPiston canister is a single vertical-cylinder flush valve that combines the function of flapper + flush valve seat + overflow into one molded piece. Kohler introduced it around 2007 to improve flush velocity and reduce leak-prone seal failures. The replacement procedure is meaningfully different from Class Five:

AquaPiston-specific symptoms requiring canister replacement

• The canister no longer lifts smoothly during flush
• Water leaks from the canister into the bowl when not flushing (worn canister seal)
• The canister has visible scale buildup that doesn't clean off
• The canister cracks or splits (rare but documented)

The AquaPiston replacement procedure

1. Shut off water at the wall, flush, sponge out remaining tank water
2. Disconnect the lift chain or lift mechanism from the top of the existing canister
3. Pull the canister straight up out of the tank — it should slide free without tools (no jam-nut as on Class Five). The canister snaps in via O-rings on the flush valve seat.
4. Inspect the flush valve seat (the bowl-side flange the canister sat on) for mineral buildup; clean if needed
5. Insert the new canister (Kohler GP1138930 or equivalent; ensure the model-specific match — there are 3 sizes)
6. Reattach the lift mechanism
7. Restore water and test flush

Cost: $35–$65 canister kit. DIY time: 30 minutes — significantly easier than Class Five since no tank removal is needed.

Common Kohler flush valve part numbers

The aftermarket question — does Korky or Fluidmaster fit a Kohler?

For Class Five Kohlers: yes, Korky 2032BP universal flush valves, Korky 100 flappers, and Fluidmaster 502P21 universal flappers all fit. The Class Five flush valve uses the US-standard 2-inch flush valve outlet, which is the same on Kohler, TOTO, American Standard, Mansfield, Gerber — universal flappers were designed to fit this pattern.

For AquaPiston Kohlers: no, the canister is Kohler-proprietary. Korky and Fluidmaster don't make compatible canister replacements. Buy Kohler-branded.

Repair-cost comparison

Bottom line

AquaPiston canisters cost more to replace but fail less often, and the replacement procedure is dramatically easier (no tank removal). Class Five replacements are cheaper but require draining and removing the tank. If you're choosing a new Kohler and want to minimize future repair effort, prefer an AquaPiston model (Memoirs, Adair, Maxton). If you already have a Class Five and need to repair, the parts are universally available — almost any aftermarket flapper kit fits.