I have replaced ballcock in the cold water tank but the overflow is still dripping. Every couple of days I tighten the cold water feed, connected to the valve. The valve cuts off but still the overflow drips.
If you are sure that the ballvalve is not dripping it can only be one of two things
The water level is to high and when the water in the cylinder is heated it expands up into the tank and overflows.
If you have central heating and the central heating tank is higher than your cold storage tank, The coil in your hot water cylinder is leaking inside the cylinder (not to unusual) the water level in both cold tanks will try to equalise and the lower tank will overflow.