The fuel tank valve is a 2 way valve to allow air into the tank so the fuel can get out and prevent the tank over pressureising.
The oil tank valve is a one way valve again to allow air in so oil can get out.
Double check by blowing into the hose and checking where the air is coming from.
While it is apart replace the impulse line ( always feel like an extra on Star Treck when I say that ) 1115 141 8600,
They get soft after a lot of use and don't power the pump as well as they should which is one reason why Husqvarna do not fit them.
Stihl are very protective of their IP so don't go putting that parts breakdown all over the web or you will get an unfriendly call from Stihl's legal team.