The air will come into the fuel lines while the truck sits unused and this is normal

you should prime the fuel upto the carb just before trying to start the engine which should help it start easier,

do this by removing the fuel line at the exhaust end and slowly blow into it until you see fuel going upto the carb,

some people carry a spare clean piece of fuel tubing with them for this purpose

dont continue to blow into the fuel line after fuel reaches the carb as this will flood the engine,

Some really small air bubbles do form naturally with vibration etc,

but if there is alot of air in the fuel line while the truck is running this maybe why it does not continue to run,

at this point you should replace the fuel lines.