u201c f u201d the force of the fuel f flowing through the valve causes the spring 32 to extend which increases the original spacing s between the ball 38 and valve seat 34 fig2 c shows valve 30 in the biased closed position which occurs when the pressure in the fuel line downstream of the valve toward engine 24 is a predetermined amount higher than the pressure upstream of the valve toward pump 14 the pressure differential that occurs immediately after the engine is shut off is sufficient to cause the valve 30 to close as seen in fig2 c this will maintain the pressure in the downstream line 20 until the engine begins to cool maintaining the pressure for this period of time is desirable since the pressure prevents the boiling of the fuel in the downstream fuel line 20 due to the heat of the recently turned off engine as the engine cools the pressure in the downstream fuel line 20 lowers at the valve opening pressure e g about 25 kpa valve 30 will open to the unbiased relaxed position seen in fig2 a and the upstream and downstream pressures will move toward a state of equilibrium conversely and as discussed above prior art fuel line valves are biased to remain closed even after engine cooling when the pressure in the fuel line begins to build back up due to diurnal heating fuel vapors.