Fuel supply system

A bypass valve (22) and vent valve (24) are connected in parallel with the normal flow path between a low-pressure fuel pump (12) and a high-pressure fuel pump (16). When a low fuel inlet pressure is detected the bypass valve (22) and vent valve (24) are opened. Gas is vented out of the fuel system and bypasses a heat exchanger (14) between the low and high-pressure pumps (12 and 16), thus avoiding the need to overcome vapour lock between the two pumps. The vent valve (24) passes gas to a drain tank (26) and then closes to prevent the escape of fuel as the high-pressure pump (16) primes.