Disclosed are a fluid control system and method for controlling delivery of two variable pressure fluids to maintain a pressure bias between the two fluids within an end use device. The system employs an actively controlled vent valve which can be integrated into a fluid control module in preferred embodiments and is actuated to an open position to decrease fluid pressure in a first fluid supply line when a determined pressure differential reversal exceeds a predetermined threshold pressure differential reversal. The disclosed system is particularly useful in a high pressure direct injection (HPDI) multi-fueled engine system where the first fluid is a gaseous fuel and the second fluid is a liquid fuel. The fluid control system and method of controlling it provide for improved control of venting along with protecting system components from high back pressure and cross contamination of fluids.