A pneumatic cushion for a wheel chair and the like is disclosed which comprises an air housing box, a plurality of air openings in the air housing box, and a plurality of air channels communicating with an air supply. The air channels communicate with the air openings to deflate and inflate individual air sacks carried over the air openings. The air sacks are unattached so that they act individually. An air distribution manifold includes a rotating blocking member to periodically block air distribution to the air sacks allowing them to deflate. The air sacks are constructed from a fabric having a low air permeability. The blocked air sacks thus allow air to escape through the sacks for deflation which cools the portion of the occupant seated on the cushion while allowing temporary pressure relief during deflation. The air sack inflation system is open so that, in addition, pressure may be relieved by a backward flow of air through the system. For this purpose, the blocking member which selectively blocks air flow to the air sacks, is in the form of a one-way valve which blocks air in a first direction, but opens in a second direction to allow a bleed-off of air from the sacks should excessive pressures exist in the sacks such as caused by an occupant's weight shift and the like.