toilet flush valve leak

[giwatcher]

I have a current model fluid master flush valve with a constant hiss after a complete flush that disappears after I turn off the shut off valve to the toilet. I have noticed the water level slowly rises in the tank after the flush is over. The level does not drop and phantom flush, which is a sign of a flapper leak. It is not a long slow flush that takes forever to fill the tank, which is fixed by replacing the valve gasket. My question is, a failed gasket has always before created a restriction in water flow, not a slow leak after flush is over. I don't want to replace the whole flush valve if the gasket is the culprit. Since I have never before had this symptom, I thought I would ask the experts before guessing.

 

[FishScreener]

The diaphragm on the FluidMasters is replaceable. Costs around four bucks. Turn off the stop. Disconnect the lever arm from the float. Rotate the cap 45-degrees, and it comes off, the diaphragm is right there. I usually hold a cup over the top of the tube, and turn the stop back on, to flush the line, before installing the new diaphragm. Reassemble, and check the float level.

 

[House Doc]

If the water level fills to the right level and doesn't overflow, you can first try some food coloring in the tank to be sure that the flapper is sealing.
I had one years ago that made me crazy and then I found that if the fill hose that goes into the tube to refill the bowl, is lower in the tube than the water in the tank, it will slowly siphon water into the bowl. It was weird but from then on I always use the plastic nozzle/clip that comes with the ballcock.

 

[TomFOhio]

Change the fluidmaster gasket. Follow the directions in post #2. The only other thing is you must pull the lever on the
fluidmaster up when turning the 1/4 turn or you will break it. Also follow post #3 on the tube going to the overflow.

 

[giwatcher]

I am able to change the diaphragm, as I have done before, when the symptom was restriction of water flow. My concern is the housing is cracked, so that water leaks from the valve body before the shutoff of the float. This would cause the tank level to slowly increase, rather than slowly decrease, as would occur with a flapper seal leak. Flapper seal is not the problem, and the tube is properly positioned and clipped at the overflow tube. I see no drip from the tube going to the overflow, which is what I would expect if the diaphragm was leaking rather than obstructed. If I hear water running, then I should see a drip from the tube to the overflow, right?
The key here is:
1) there is normal water flow, which has never been a symptom of diaphragm failure before.
2) tank water level increase over time. It does not drop.
So specifically, has anyone ever experienced my described symptoms from a failed diaphragm?

 

[FishScreener]

It happens when a bit of crud gets on the seat.

 

[giwatcher]

Thanks for the reply. Fluidmaster is sending a warranty replacement fill valve. No debris on seat, flush x2 and still runs. Internal valve defect.