Unimog Club UK

77 u900 should be a lever right next to your leg pull it up two clicks you should hear air valve clicking.ten inch chipper is big for a u900 it will drive it but don't push it to hard as the pto box and clutch spline will suffer.
Ready your post your sticking brakes need sorting don't drive it with the brakes dragging as you'll cook the hub oil and that will destroy the hub gears very expensive to fix.
Moose