My 5 ton does the same thing. After sitting for a day, it takes an act of congress to get it to fire up. The pump is working, good clear flow at the bleeder on the filter. Once it starts, it seems to be good for the day. Haven't really taken the time to work on it, but it sure acts like it's getting air in the fuel system. I'm planning on checking all the fittings and lines and if that's not it then maybe replace the injection pump itself. One other thing I just thought of is putting a check valve in the fuel line like the old Detroit Diesels used. Might try that first.