A vehicle air braking system having a source of fluid under pressure (1), an air dryer (A) downstream of the source (1), and an air consumer circuit (2) downstream of the air dryer (A), the system being operable to suspend the source (1) on demand to permit backflow of air under pressure from the air consumer circuit (2) through the air dryer (A) to a drain valve (B), the system including a regeneration valve (D), and a command valve (C) having an inlet connected to the air consumer circuit (2), and a supply outlet operably connected to the drain valve (B), wherein the regeneration valve (D) is operable to permit regeneration backflow in response to connection of the inlet and supply outlets of the command valve (C).