http://www.toiletology.com/howtofix.shtml

How about trying the following

1- If this is a low down tank remover the cover and with a flash light look down the over flow tube and look if any water is flowing.

2- Shut the water to the tank and add instant coffee or food dye and see if it shows up in the bowl.

3- Check the ball c o c k to make sure that it is not adjusted to high allowing water to flow over the refill tube , check the float.

4- If your using a fluid master 400 A, make sure you have a loop above the fill valve to prevent any siphon

5- If you lift the tank bulb or flapper feel around the Douglas valve to make sure the seat is not pitted.

6-Check the flapper / tank bulb / Disc depending on brand

7- If this is a flushometer shut off the water supply dissemble and check the piston or diaphragm assemble and clean out the by pass opening with a pin.

Reassemble and test

If this doesnt work get back to me

"4- If your using a fluid master 400 A, make sure you have a loop above the fill valve to prevent any siphon"

It take more than a loop.

The discharge end of the tube needs to be ABOVE the water level line.

Don't matter if there is a loop up or down as it filled with water when filling and if the end is below the water level line it will continue to siphon.

DAMHIKT!

you are a billion perfect correct as usual