A control valve for a reverse osmosis water purifying system provides a feed water port, a squeeze water port, a drain port, and a product water connection, each of which open into a bore. A first, second, and third O-rings are located in the bore successively between the feed water port, the squeeze water port, the drain port, and the product water connection. A control piston is moveably located in the bore of the housing. The control piston includes a vent/drain well in which a side is chamfered. The vent/drain well provides a fluid passage between the squeeze water port and the drain port when the vent/drain well passes over the second O-ring. The fluid passage includes an opening formed between the second O-ring and the chamfered side of the vent/drain well, where the size of the opening is responsive to the position of the control piston.