Abstract: A control valve having relay and proportioning pistons has an intermediate piston which controls air pressure access to a blind flow chamber. At a relatively low value of control air pressure, the intermediate piston is shuttled into a position which closes and traps the existing value of control air pressure in the blind flow chamber. The trapped constant value of air pressure in the blind flow chamber operates on a larger area piston whose force is added to the forces derived from further increases in control air pressure to provide a first region of output air pressure having incremental increases smaller than the input. When the input air pressure reaches a certain value, a second region of output air pressure is reached where further increases in input air pressure cause metered addition to the air pressure in the blind flow chamber and thus produces incremental output increases greater than incremental input increases.