Between Omaha and Des Moines, 7 miles South of I80 | Ok, that is the by-pass "one-way check valve" that allows a person to use the "hand primer pump" ; because engine/fuel pump uses a gear type fuel transfer pump, thus the fuel needs a passage to "by-pass" the transfer pumps gears(non-running), so the hand primer pump can deliver a reasonable volume of fuel, while hand pumping. The hand priming pump, also has 2- "one-way check valves", that should "stop" any drain back, if they are working correctly. 
Edited by 4WD 4/12/2017 20:12
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Fuel leaking back to tank on a 3208 Cat
