Combines MF750 hard to start

The lift pump is bad and letting air in when you shut it off. However ANY AIR lEAK between the tank and filters will let fuel run back to the tank.This was a problem I would see alot and most of the time that was it. good luck

The first thing I checked was the lines and pumps from the fuel filters to the tank. With the valve at the tank shut, I can pull and hold a vacuum with my hand operated vacuum pump - no leakage at all. I can do the same for the fuel return line (with the line plugged at the tank). My new electric fuel pump easily supplies all the fuel for voluminous bleeding. The new lift pump also works well by hand, though I think it is bypassed by the capacity of the electric pump.

Whats the history of the fuel injection pump it might be toast.

Since I've only had the combine for 2 seasons, I do not know the history of the pump. The combine engine behaves well, (giving excellent fuel economy),except for the air lock starting problem when left standing more than a few minutes. Having the pump checked is my last resort because of the cost. The lowest quote (out of 6 shops) I have been able to get for an injection pump checkup is $1200 +. While starting to disconnect the pump, I noticed that one of the lines going from the filters to the injection pump was somewhat mangled in the area of the flare (suggesting a previous pump service),and that the rubber seals were badly deteriorated. I'm changing that line and all the rubber seals in the line connections this morning. I'll let you know what happens.

Problem Solved! I replaced the little rubber sleeves at the end of all the fuel lines in the area of the filters and injection pump. The 5_16" lines use part number 376 526X1, while the small return lines use 376 524X1. These rubber sleeves come in pack of 10 from Agco. It was quite finicky chipping out the hardened and brittle old rubber sleeves from the fittings. I removed the fuel filter assembly and the fittings on the injector pump to get at them. Sure was a lot cheaper than having the injector pump and injectors serviced, and then changing the rubber sleeves.