I had a toilet leak at the supply line toilet connection. When I replaced the supply line, I still had a leak at the connection. I then replaced the fill valve and that solved the issue. The plumber told me that there was debris in the fill valve that caused damage and caused the leak. I am puzzled how debris in the fill valve could caused adverse pressure on the supply/fill valve connection; but no doubt the leak was coming from that area. Can damage in the fill valve cause a back pressure condition on the connection?

If there is debris inside the fill valve the washer inside the fill valve will vibrate during the fill operation or at the end of the tank filling, you will hear a loud sound and it causing vibration and may cause the supply line to leak if it is loose, but let's wait what will the other experts has to say, Thanks.

John