Reservoir tank continues to slowly fill and won't stop
Most modern fill valves you can turn off the water and remove the cap. Twist the upper valve part and it will unlock showing the seal and mechanism.  Place cup over valve tub slowly turn water to toilet on and run 3 seconds and turn off.  Reassemble valve by aliging tabs and twisting back on its only a 15 or 20 degree twist so be gentle. Turn water on and test.  If you live in a hard water or silty area or just have old galvanized pipes flakes get caught in there all the time.  I was buying new valves often till I figured this out.  Half the time a valve is leaking it's usually this I have found and you can't imagine the clients that are shocked to just get billed a minimum service call that took under 3 minutes and most of the time if they are close or in my current route of the day I usually don't even charge them nothing more than a bottle of water or a cup of coffee ☺