You can have an air leak in the system without seeing a drop of fuel. It'll purge just enough air into the system when sitting to make it a b!tch to start. It's possible for this to happen even in the sending unit in the tank. So, for instance, if there's a crack at 1/2 tank level, it'll never give any troubles untill parked overnight at less than 1/2 tank, OR, parked on an incline that exposes the cracked line.