I have several W.Cs and two of them continually pass water from the cistern into the bowl. A very small flow only visible to the eye when the bowl is lit from the bathroom lights.

I am quite handy, with an engineering background, and I have exhausted my knowledge trying to fix these.

Either water is leaking passed the flapper valve when the cistern is full or water is leaking through the overflow.

I have adjusted the ball float so that the water level in the cistern is correct (no overflow).

I have removed the flapper valve and carefully cleaned the sealing surfaces.

I have cleaned the rim of the cistern outlet on which the flapper valve seats.

I have tried adjusting the height of the plastic float that is clipped to the chain connecting the flush lever to the flapper valve.

I have tried a new flapper valve.

All to no avail. I don't have any problem with 5 other WCs using the same valve system.

Continous leak-through wastes water and could flood the septic tank.

I think that it can be fixed by 'tweaking so, are there any DIY plumbers out there who can give me any tips?

Can we assume that the even if the wc is not used that occasionally the water level in the cistern lowers enough so the float valve opens and refills the tank?...or can we assume that even though the water level is set accurately that a leak in the float valve let's water to slowly run into the cistern so that it eventually overflows the overflow riser?

Summary...determine if the float valve cycles when the toilet is not in use for long periods of time.

Chonwah

Have you checked the connection from the bottom of the cistern to the bowl? There could be a leak there

Thanks guys, I had tried all that you say. Having written down the problem yesterday I sat back and reviewed all the steps again and went and physically double checked the actions and found the cause.

All of my WCs are COTTO with standard operating mechanisms all set up the same when compared one to the other but, the two problem WCs I now find have a small (2mm) hole drilled through the vertical overflow tube a few millimetres below the collar at the top. The holes, in both cases, are not visible from the front but are in the rear arc of the tube. The hole is just below the waterline when the water level is set so that the inlet valve shuts off when the level is about 1 cm below the top the tube. So, a small trickle of water is constantly passing from the cistern into the bowl.

Solution? Block the hole or lower the water level slightly. I chose the latter and the problem is cured.

Another of life's mysteries! Why holes in these two and not the others? What purpose does it serve?

OK. Nuff said, now on to the next problem............

I suspect the hole is designed to catch the hose clip that directs the incoming water and was not designed for the model toilet you have (internals are sold separately). As your water level is now a bit low be sure they still flush OK.