A pneumatic valve for an agricultural product distribution system includes a fan inlet, a tank outlet, a shuttle, a control pressure inlet, and a diaphragm. The fan inlet is coupled to an air source that supplies an air stream into a primary line, which facilitates distributing a granular product to an agricultural implement by guiding the air stream from a first section to a second section of the primary line, the first section having a larger cross-sectional area than the second section. A tank outlet is fluidly coupled to a storage tank. The control pressure inlet is fluidly coupled to the second section of the primary line or to a meter housing. The diaphragm has a first side that is coupled to the shuttle and is exposed to static pressure in the storage tank and a second side that is exposed to the static pressure in the second section of the primary line or the meter housing, and aligns a shuttle hole of the shuttle with the fan inlet when the static pressure in the storage tank is not greater than the static pressure in the second section of the primary line or the meter housing by a threshold amount.