Can't fix a leak. HELP !!

I woke up one morning and my toilet tank (4 years old) was dripping water from the area where the water supply line goes in. I thought this must be a rubber washer issue so I replaced the rubber washer inside the tank around the ballcock shank. I connected water supply and turned water on. No success. Now I am getting even more water leak. I then replaced the water supply line which comes with rubber washers. I connected again...same thing. If I fill the tank ,with water line disconnected, there is no leak. I get the leak only when I turn water on and the tank starts to fill. This tells me it has to do with water pressure of some sort, or water getting obstructed when it enters the tank ? But the tank fills ok. I tightened both the mounting nut on the shank under the tank and the water supply nut at the shank. I have a diaphragm ballcock. Can anyone give any clues why water gushes when I turn water on ? I'm pretty handy but this one got me good !!


The only thing left to do is change out the ballcock assembly itself.There could be a flaw of some kind in the ballcock possibly a hairline crack in the plastic that only leaks under pressure and/or is being opened by the tightening of the nuts.

Also examine the porcelian all around where the ballcock passes through the toilet tank for hairline cracks or flaws in the glaze etc.

That is all that's left that you can do.

If you change the ballcock get a Fluidmaster 400 which just about any hardware store etc will carry.Has a great reputation,is easy to install and maintain.If the supply line you now have has become worn by the tightening you have done replace it.