Abstract: The invention relates to a drain valve and a method designed to drain condensate and contaminants from a wet tank in a pneumatic braking system for a vehicle. In a preferred embodiment the drain valve comprises a cylindrical piston disposed to slide axially within a cavity in a valve body constructed to be mounted on the vehicle and having two openings in the side wall of the valve body communicating with the cavity, one, an inlet opening connected upwardly to the wet tank, and, two, an outlet opening connected downwardly as a drain. The outlet is blocked in a closed, first position of the piston when the vehicle ignition is turned on. When the vehicle ignition is turned off the piston moves from the first position to a second position so as to open both the inlet and outlet openings together through the cavity in order to drain the wet tank. When the vehicle ignition is turned to the "on" position the piston moves back to the first position within the cavity so as to close the drain valve.