Heres what to do. If the waters running and wants to fill the reservior then its the cap that lays on the top of the exit hole. When you push down on the handle, the handlerod thats connected to the chain, and then its connected to the stopcap. Replace the cap with another one, don’t buy the cheapest because they oxidize too quick and become brittle and begin to leak. Buy another one for a couple bucks more because their made from a more pliable material and seal longer. Its an easy fix just take your time and don’t pull on anything hard, just pull the old cap off its mount, via fingers, and replace. This should solve your problem, I’ve fixed dozens of them.