A control valve is provided for a fluid treatment apparatus, including a housing with at least one cylinder defining a fluid flow path in the apparatus, each cylinder having an associated reciprocating piston valve. The valve includes a first piston valve half; a second piston valve half complementary to the first piston valve half; and a resilient seal constructed and arranged for being sandwiched between the first and second piston valve halves. The first piston valve half is constructed and arranged for engaging the cylinder before the second piston valve half, and having an exterior peripheral edge defined by a plurality of peripherally spaced teeth creating flow spaces between the teeth.