Petrol tank building up positive pressure, is this ok?

[Boerbok]

My bike is on 6000km, and runs just fine. But here is something I noticed recently, if I open the petrol cap after a ride, I get a hiss of air blowing out.  :eek7:

With the cap open, if I blow or suck on the tank breather, I can get some air flowing in and out, so the breather seems to be open.

Is this normal for the tank to build up positive pressure? How is this even possible if the breather is actually breathing?

 

[max lange]

 

[Boerbok]

 

[ianb]

 

[Mikerider]

 

[the real Takbok]

 

[lecap]

 

[Boerbok]