As robotic systems enter an ever expanding spectrum of industries, it is becoming increasingly important that they are built to respond quickly and efficiently to dynamic environments. This letter presents a passive, reflex response (PRR) unit for quick actuation of pneumatic soft robotic systems. The PRR unit consists of a pressure reservoir with a soft one way valve that can be opened by passing a pin through its aperture. The device is mechanically stimulated by the environment, leading to the release of stored pneumatic pressure into the body of a soft robotic system. Prototypes of the PRR unit were characterized with three different valve sizes (6 mm, 9 mm and 11 mm) across a range of reservoir pressures (0.2 bar to 1.2 bar). A pin, attached to a load cell, was then lowered into the valve aperture until it opened. Force and pressure dynamics were recorded, along with the curvature of a pneu-net bending actuator during successive testing of the unit when coupled with a soft structure. For all valve sizes, the force required to open the valve increased proportionally with increasing internal pressure, with a minimum force of 0.98 N and a maximum force of 4.68 N. Time to vent the system decreased with increasing valve size, from 1.30 s to vent the system with the 6 mm aperture at 1.2 bar, to 0.53 s to vent the system with an 11 mm aperture. When integrated with a pneu-net bending actuator, the force required to open the 11 mm aperture increased to 15.82 N when charged to 0.5 bar of internal pressure due to the added force required to push on the constraining layer of the pneu-net. The PRR unit actuated the pneu-net in 0.37 s when charged to 0.5 bar of internal pressure. This demonstrates how reactive soft robotic systems can rapidly respond to mechanical environmental stimuli without the need for high-level control.