I am working on a 2005 IC RE DT-466. Having a tough time starting because of air in the fuel. Have replaced the lift pump and made sure the fittings were tight. I can get the bus to start if I prime the fuel system but eventually more air gets sucked in and it dies. Made sure the fuel filter was changed and the cap on tight. Also, did the same for the strainer. Icp pressure is good when bus is running with fuel pressure at 68 psi. Could use any suggestions. Thanks in advance.