[FrankJ]

The valve requires power is operate. As soon as it looses power all internal valves close. (per data sheet). So with a simultaneous loss of control signal and power, you might see it start to vent, but is would quickly fail closed. The actual pressure would depend a lot on the volume of the downstream side

Actually the data sheet shows that it is not relieving regulator so that specific model shouldn't be legal as a primary regulator. The relieving function requires active control of the regulator. The power failure mode (main switch off) would be to stop controlling presssure & not release down stream pressure when upstream is vented and vice versa preventing proper operation of a single point vent valve. (Maybe this would be a place for clever use of a check valve to allow the downstream to vent to the upstream side.) Another complication is the 24 VDC supply voltage. That would mean powering it through the PCM, which disables when robot is disabled or a custom circuit.