toilet tank losing water

HI, the toilet tank is losing water into the bowl, causing it to refill the tank every 20 mins or so. I have checked the tank valve, it is reasonably new (under 2 years) so i dont think it needs to be replaced and it seems to be seating properly.
I have also checked that the chain leading from the ballcock is taut (it is) and the water level in the tank is high enough. I had replaced most of the innards of the tank over a year ago and it has been working with no problems up until now.
Any other suggestions? Thanks,
corey

 

Only two ways for the water to get from the tank to the bowl - past the valve seat or through the overflow.

Two things:

1. Replace the flapper valve, they're very cheap. While you have the flapper off, clean up the flapper seat with a very fine abrasive pad (Scotchbrite, etc; not sandpaper).

2. The chain to the flapper should be slightly SLACK, not taut. If it is taut it is probably allowing some seapage past the flapper.

Good luck.

 

The ballcock assembly that controls the flow of water also has a small tube that sticks down the throat of the 1" overflow tube. This tube delivers water to the bowl of the toilet and refills the bowl as the tank is refilling after you flush. Sometimes if the tube is stuck way down inside the overflow tube (below the water line in the tank) you will get siphonage and water will constantly trickle down the tube. Sneak up on the tank when it is done filling and look down the 1" overflow tube with a flashlight. if you see water running down the side of the tube then you need a 99 cent airgap clamp. It is a plastic "L" shaped tube that clamps to the top of the overflow this holds the overflow tube in the air above the water line and breaks the siphonage.

(sorry for the wordy explanation. Can someone teach me how to do the cool picture thing?)