maxum 2300SCR water tank air exhaust issue

[baza1978]

Hello.
there is a liitle problem with my water tank.
filling pipe is placed about 2/3 of tank height, and seems there is no any air outlet valve. So i cant fill full tank, because there is no way out for the air.
May be there is a "secret" valeve which i hadnt found?
its not my tank on photo, but mine 100% the same.