fuel supply for a petrol grass timmer?

I'm trying to fit new petrol supply tubes to replace cracked and leaking ones but I've got hem mixed up. There's three tubes - two come and go to the petrol tank. the third goes to the carb (?) There's a rubber primer button that usually shows a little petrol in it when I press. Could I have the flow the wromng way or could the thirdtube be also fitted wrong?