My toilet fill valve leaks at the top only when the bowl is full. Until the bowl is not full it is not leaking from the top.
This is a video of the leak: https://vimeo.com/270998266
I want to decide whether I should only replace the cap of the fill valve or the complete fill valve. I ask because the cap is 10 bucks cheaper. Should I replace the cap or the complete fill valve? Is there any test I can do to figure out this myself if this information is not sufficient to decide