In the valve assembly for an ion exchange water softener as disclosed in the parent patent, the piston controlling the reversing valve in inlet-outlet by-pass is replaced by an elastic membrane associated with a rod mounted on this valve and able to be pushed by the membrane into the posn. in which the by-pass is opened. Specif. the membrane is associated with a disc and is clamped at its rim. The disc has a central rod on its underside which engages a bore in the valve rod of the reversing valve which opens or closes the by-pass connecting the raw water fed to the exchanger outlet.