air leaking on handbrake iveco 75e15?

hi,wonder,if anyone can help? just had new air pressure dump valve fitted and on way home noticed i have a leak /hissing from handbrake area, only when handbrake is off,as soon as handbrake is moved to on position hissing stops...new handbrake req? or something simpler ?? air pressure good does go down from time to time but mostly stays just under 12 bar and "blows " when required