One of the Niagara toilets would not fill but just kept running so Dave took it apart.
Dave figured out the problem. He said most internet sites dealing with the tank not filling blame the flow valve. This (fixable) problem was in the dual flush mechanism.
In its normal, closed position, the push cap is up waiting for the command to flush. The middle piston connected to the lower piston/valve is down. When the flush button is pushed, the push cap goes down, raising the middle piston and the lower piston/valve. The lower valve opening allows the water to go out and flush.
When the button is released, the top piston, attached to the lower valve, depends on gravity to have it drop and close the valve.
Unfortunately there was mineral build up in the valve preventing the top piston’s fall. The valve stayed open and the tank could not fill.
The fix was to soak the assembly in vinegar to clear out deposits that were keeping the valve from falling into place.
Quite a bit of mineral buildup was cleared from the valve.
Once the valve was fully cleared and slid up and down freely the problem was fixed and Dave reassembled the toilet.