Mv3 valve

Hi everyone!
On my KW W900l I have air leak If I push in yellow button it start leaking but when I press break pedal it stops. I check the line and it's going from mv3 valve outside behind dashboard. after awhile I put plastic seal on it so after it's dried I tried push in yellow button again to check air leak so it's make pressure inside mv3 valve and I can't release truck breaks.
What it can be because it makes me pain!
Thank you!