Challenger MT455 Fuel supply problem

I have owned this machine since new. One day recently it simply would not start, so I started looking for the problem. I opened the injector connections and none were delivering fuel. When I operated the fuel lift pump manually, it was delivering fuel mixed with air. Considering the age of the machine, I decided to go through the fuel system starting at the tank. I removed the fuel pick up and found the filter sock was missing, so I purchased a new one and installed it. Also found the fuel float had a leak, so I replaced it while the tank was open. Having done this, I reconnected the fuel lines and again attempted to prime the system. In doing so, it still produced fuel with air. My thoughts were either the fuel lines are leaking or the lift pump diaphragm has failed, so I replaced all rubber fuel lines, the fuel pump, and hose clamps, finding in the process an original plastic hose pinch clamp had failed on the suction side of the lift pump, allowing a path for the entry of air. While the fuel supply system was opened, I cleaned all the fuel filter elements and replaced the water/oil filter with a new one. Once the system was primed with air free fuel, I retested the system with all injectors open. None would deliver fuel, so I moved on to the fuel shut off solenoid. I replaced the old solenoid with new. This time testing the open lines to the injectors produced fuel, so I put it back together and it immediately started. I used it for about 3 hours and it was performing fine when I shut it down. The next morning, it would not start, so I presumed there was a defect in the new solenoid and replaced it with another new unit. Once again it started promptly and performed fine for the day until I shut it down. The next day, again it would not crank. I remove the solenoid again, and tested it through the wiring harness of the tractor. It would not actuate, so I removed it from the harness and went directly to a fully charged battery. Neither the first nor second solenoid would actuate while directly connected to the battery but both did get hot. When I touched the plunger with my finger, it would actuate. It was then that I supposed the solenoids were not suitable for the Delphi pump I had, so I order from another source and from another manufacturer. BTW, both new solenoids read 11.7 ohms of resistance at the end of each battery test. This tractor has two batteries and the harness shows 12.7V with the switch activated. As Einstein was quoted, "doing the same thing over and over and getting the same results defines crazy. Three new solenoids have not corrected the problem. The solenoid manufacturer's say it is an electrical supply problem. I have ordered a new ignition switch just in case, but the voltage reading of 12.7V at the connectors to me, does not seem to be the problem. Has anyone else been through this before? The Injector Pump is a Delphi type 1272. Thanks in advance for your understanding considerations in this regard. Kirk Nivens