Replace flush valve gasket k3386 - Kohler K-3386-0 Rialto one-piece round-front toilet with seat and left-hand

Take the worn item to a plumbing supply store along with the details of the toilet model.

replace the whole valve. It's cheap and easy to do. Probably you can install a universal valve. Look on e-bay or amozon

I’m happy to help further over the phone at https://www.6ya.com/expert/robert_24775b73ca70da93

You need to replace the flush valve gasket. It comes in a flush valve kit, it's a rubber gasket that goes between the tank and the bowl. You just pull the tank off and it's on the under side of it. Be careful the black stuff on the gasket doesn't come off your hands. Make sure you get the new one on there even and all the way on. This will fix your problem. Good Luck

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If you have a supply shut off to fixture shut it off and disconnect supply line from inlet to toilet . If your supply line to toilet is a flexible line then bend it so that you can direct water to small container. Slowly turn valve on to see what flow rate is at that point. If flow rate is not good then I would shut water off to house and replace valve. If flow rate to toliet is good I would reccomend you replace flush vallve. To do this shut off water supply to toilet. Disconnect supply nut to bottom of flush valve. There will be a retaining nut that holds flush valve in place in toilet tank. Remove flush valve after removing fill tube from flush valve to overflow tube. Replace with toto replacement valve or use Fluidmaster 400A. Instructions on box of 400A are easy to follow and if you have flex connector you won't need supply line washer. I would reccomend that before you install new valve that you put rubber gasket on shank of valve and slid it about halfway up the shaft. Make sure that inside of tank is dry and use a small bead of silicon caulking on top and bottom side of shaft gasket. slid gasket the rest of the way on valve shank,install in tank and put on retaing nut hand tight. Push down on top of valve and using channel lock pliers finish tightening nut securely.You may need to reach into tank and grasp valve body to stop it from turning. There are other things you can do if you have a working supply valve. Shut off valve and remove screws that hold top of valve in place and remove and clean surfaces. You can turn valve on slowly to help flush out any rust or mineral deposits. I hope this is of some help to you. Thanks

The top of the pipe above the flush valve is threaded if you unscrew this you will be able to remove the flush valve and replace the gasket. I am assuming we are talking about the same here is a link that has all the parts:

http://www.plumbingsupply.com/kohlertoiletparts-santarosa.html

The top of the pipe above the flush valve is threaded. Unscrew it, and you will be able to remove the flush valve and replace the gasket.

First things first, remove the fill valve, and the flush handle mechanism. This will give you plenty of room to work with the flush valve. Before installing flush valve lubricate tabs with plumbers grease. Now put on a pair of work gloves, pushed down very hard and turn the tabs should slide easy because of the grease. Remember do not alter the gasket, the gasket needs to be compressed to do its job. I know removing the fill valve and the flush handle seems like a pain, but there is no way to get your hands in the proper position to install the flush valve assembly.

You will need to replace or repair with new gaskets your flush valve mechanism. "To do that you need to take the tank apart from the bowl. Have new toilet bolts and flush valve or flush valve gaskets at the ready when you take the tank off. (you will also have to unhook the fill valve when you take the tank off.) Another possibility would be a cracked tank that is causing the leak in that area but that is rare.

try a plumbing supply co. they may help you

You have replaced the flush valve completely? If so and the unit is still leaking, you may have a small crack in the tank or your just not getting the tank to bowl fittings tight enough. Also, they make two different thicknesses in the tank to bowl gasket. Make sure you are using the thicker of the two. You posted this under the American Std. thread, so if this is correct, give them a call. There could be a recall on this particular toilet due to a flaw.