Got a 5400 John Deere tractor, about a 1998 model. About a year and a half ago it would all of a sudden, just die. Let it set a few minutes and it would start back if you pumped the primer button on top of the fuel filter housing. Bought a low pressure electric fuel pump and plumbed it between the fuel tank and the fuel filter. Worked great until 3 days ago. Same old problem except this time I am getting air from the bleed screw at the filter housing. Bleed some air off and it will start right up and it may run for 4 hours or it may run for just 10 seconds. All clamps are tight, no leaks, pump is pumping. What's going on or what have I missed?