My toilet shut off valve leaks but ONLY while the toilet is filling up after being flushed. I've looked at numerous articles on how to repair the valve but I'm not sure what part exactly is broken. What part is it mostly likely to need replacing if it's leaking only when the toilet is filling up?

Show us a pic of exactly where it is leaking.

Hi Fdts70

It could be the supply connection to the toilet.

It could be the supply connection to the shutoff.

It could be the packing nut behind the handle of the shutoff.

It could be the toilet tank fill valve nut located just above the supply connection at the tank.

Could probably continue with even more "could bes"... ;)

Here, you have no choice, you need to isolate the leak source, right? Take some tissue paper and dry everything up real good... the connections, the shutoff handle, behind the handle, etc... everywhere, OK? Once you are sure all is dried flush the toilet a few times and then use the toilet paper again, at the highest point first (at tank fill valve nut), and work your way down to every connection until the toilet paper gets wetted... that should isolate this for you. You especially want to check the supply hose connections and if the supply hose is a flexible braided hose check the crimps of the connectors as well... leak here quite a bit.

Let us know what you find and we can advise, if needed.

Mark