An air dryer for an air braking system of a vehicle, the air dryer comprising a mounting (20), and a removal e desiccant canister (10) screw-threaded to said mounting (20) about a spin axis (25) and having a peripheral sealing ring (13,14), wherein the mounting includes an upstanding protrusion (28) radially spaced from said axis (25) and adapted to engage an annular recess (17) of said canister (10) within said ring, said protrusion (28) being hollow, and the interior of said protrusion (28) being connected to a vent for air under pressure in the system.