VP getting air in it?
 

One of our trucks starts a little hard after it sits overnight, just started doing this a few days ago.

Lift pump runs fine and fuel pressure is good, but the engine cranks for several seconds 4-5 before starting, sometimes it will start but act like there is air in the system.

It seems to me that there is a check valve in there somewhere but I dont remember where.

Could an injector bleed back and get air in the lines that way?