Water gushing out of the overflow pipe is usually a sign that a ball valve is not functioning properly. Inspect the appropriate cistern.
Perhaps the ball may be lying partially or fully submerged in the water and thus not raising the arm to shut off the valve when the water has reached the “full’ level in the cistern. The reason is probably that the ball has become punctured. Rusting can make holes in it. Repairing it is not really worthwhile. Instead, remove the damaged ball — it screws on to the threaded end of the arm—and replace it with a new one.
To stop water from pouring into the cistern while you are carrying out this or any other replacement or repair, turn off the appropriate stopcock to isolate the cistern. If there is none, place a stick across the top of the cistern and use string to tie the arm to it in a raised position. You can use this ruse to shut off water to the cistern overnight if a repair is beyond your capabilities and you are waiting for a plumber.
Re-washering a ball valve.
It may be that, even with the ball and arm fully in the raised position and the valve apparently closed, water still drips into the cistern. In that event, the valve probably needs rewashering or dirt is obstructing it. To re-washer, first shut off the water supply, then:
1. Detach the lever arm of the valve by removing a split pin at the pivot point.
2. Take out the piston; you may have to prise it out with a screwdriver.
3. Unscrew the two parts of the piston.
4. Remove the old washer.
5. Wash out the valve to remove any particles of grit and smear vaseline over it.
6. Fit a new washer.