Finally stopped my toilet from ghost flushing every ten minutes

I ran into this exact same headache with an older Kohler Cimarron in my guest bath. I went through three of those red Fluidmaster flappers in a single year, convinced that the chlorine in our city water was just eating the rubber. Every time I swapped it, the ghost flushing whould stop for a week and then slowly creep back until I was hearing that hiss every twenty minutes. Instead of the glue-on repair seat, I tried a slightly different route. I drained the tank completely and used a small square of 400-grit wet/dry sandpaper to lightly buff the rim of the flush valve. There was a tiny buildup of calcium—felt like a grain of salt—right where the hinge meets the seat. Just a few minutes of sanding made the plastic feel glass-smooth again. It’s been holding steady for nearly ten months now. If anyone else is dealing with this, definately run your fingernail around that rim before you spend more money on parts. If you feel even the slightest bump, that's your culprit.

I spent way too much time scrubbing the valve seat before I realized my problem was actualy the refill tube. On my Mansfield unit, the small rubber hose that clips to the overflow pipe was shoved about three inches down inside the pipe. I didn't realize that if that tube sits below the water line in the tank, it can actually create a siphon effect. It slowly pulls water out of the tank and dumps it down the overflow drain til the float drops enugh to trigger the fill valve. It took me forever to catch it becuase everythign looked fine when the lid was off. I eventually noticed a tiny ripple in the overflow pipe while the toilet was just sitting there. I pulled the tube out, clipped it so it was spraying from above the pipe rather than sitting inside it, and the ghost flushing stopped immediatly. It’s a five-second fix that usually gets overlooked when everyone is focused on the rubber seals or the mineral deposits.

Back in the day, we didn’t always have these fancy glue-on kits. If you’ve sanded the seat and it still acts up, check your lift chain. I’ve seen plenty of guys set them too tight, so the flapper never truly settles. You want just a bit of slack so it can drop heavy. Don't forget the old dye test either. Drop some food coloring in the tank and walk away. If that color shows up in the bowl, you’re still leaking. Sometimes the leak isn't thru the valve at all, but seeping through the tank-to-bowl bolts if the washers have rotted out. Seen that happen plenty on those old heavy-duty units.

While mechanical seat integrity is paramount, one must also scrutinize the potential for lateral torsion within the flush lever assembly itself. In my own experience with a high-efficiency Toto unit, I discovered that a slightly misaligned trip lever was causing the flapper to descend at a marginal angle, thereby preventing a perfectly concentric seal despite the surface being ostensibly smooth. If the brass or plastic arm possesses even a minute amount of play, it can introduce enough deviation to allow for a microscopic gap that regular sanding will never rectify; ensuring the nut on the handle is reverse-threaded and sufficiently snug is often the overlooked variable in this hydraulic equation.

Glad that worked for you, but have you ruled out the tank-to-bowl bolts yet? I chased a 'ghost flush' for months and assumed it was the flapper seat too. I even tried the sanding trick. Turns out, the rubber washers on the bolts at the bottom of the tank were just crusty enough to let a tiny bit of water weep into the bowl thru the bolt holes. It looks exactly like a flapper leak. If you're still hearing a hiss later on, I'd dry those bolt heads and see if they're actually staying bone dry.