The leakage guard is formed as an integral part of the mat with a freely movable valve ball which is dimensioned and arranged so that the pressurised air can freely move into the mat while return flow of water against the upper stub opening of the inlet stub presses against the ball to force it closed. The air is fed into the mat through the guard so that the bottom end of the inlet stub has a valve seat (1b) so that forcing of the ball through the opening and into the mat interior is prevented.
