Linear motion fluid-actuated valves in which all elements are in rolling contact. Two major rollers are prevented from moving apart by a retainer belt, a separator roller depressing the belt between the two major rollers to prevent them from moving towards each other; two inner, opposite sides of the rectangularly cross sectioned valve-body holding the separator roller in position with respect to the retainer belt, the other two sides being in contact with the ends of the assembly of movable elements. Admission of fluid into an end of the valve body causes the assembly to move and thus expose a port in the valve body. Accumulators, pressure relief valves and fluid flow control valves are among the useful applications.