Help please!

[Hefner]

The cold water overflow in my loft is overflowing. I have previously bent the valve arm back to prevent the ball rising as high. The problem has returned and upon checking the cistern the ball in completely submergered. Does this mean that there is a faulty with the ball cock and it simply needs to be replaced?

[oilman]

It might do. Then again it might be an internal leak in the hot water cylinder.

[plumberman_uk]

very likely the washer on the ballvalve has perished...dont try repairing it if the valve looks old.. better replace the lot

[why me!]

you might want to check the piston valve, if its cracked (it only takes a small hairline fracture to cause a leak) replace it, they're only about £1 or at worst replace the whole ball cock arm mechanism (about a tener)