pressure responsive force transmission apparatus such as may be used in automatic controls and for hydraulic and pneumatic lift or force mechanisms comprises a pair of substantially rigid lift elements each having a base plate , at least two guide arms projecting from the base plate , and at least two guide surfaces at the sides of the base plate . the pair of lift elements is mated together each with its guide arms engaging the guide surfaces of the other . a flexible inflatable bladder is disposed between the base plates of the mated lift elements . one of the lift elements is formed with a fluid transmission passage communicating with the interior of the bladder . by delivering fluid under pressure through said passage , the bladder is inflated and causes the lift elements to move apart .