Abstract: A booster compressor system modulates discharge pressure with a proportional regulator in fluid communication between the compressor discharge and a servo control for a valve to the compressor inlet. The servo control comprises a double acting piston connected to a valve control rod. In one embodiment, the proportional regulator is of a negative signal type and communicates to a side of the piston for urging the piston to a valve opening position. A valve closing pressure is applied by a non-relieving regulator in fluid communication between the compressor discharge and the other side of the servo control piston. As the discharge pressure increases, the proportional regulator out let pressure decreases thereby resulting in the servo control allowing the intake valve to the compressor inlet to close. This, in turn, reduces the compressor discharge pressure thereby resulting in the proportional regulator controlling the valve to partially open once more.