Low friction seal assembly

A fluid energy translating device comprising a housing, a member movably mounted in the housing and dividing the housing into first and second fluid-receiving chambers, fluid supply means for selectively supplying high pressure fluid to the first and second chambers, a seal assembly carried by the member to seal the first and second chambers from one another, the seal assembly having a pair of back-up plates, a spacer positioned between the back-up plates, an O-ring fitted onto the perimeter of the spacer, and a seal member positioned between the back-up plates which engages the walls of the housing, the spacer being thinner than either the seal member or the O-ring, the O-ring positioned in an opening in the seal member, the O-ring mechanically squeezed radially between the spacer and seal member to bias the seal member against the housing walls and squeezed laterally between the back-up plates, a plurality of shallow notches formed in the perimeter of the spacer to provide additional spaces for the O-ring, fluid passages in the back-up plates for conducting high pressure fluid between the back-up plates to hydraulically balance the spacer and to hydraulically balance half of the O-ring in the radial direction and half of the O-ring in the lateral direction, and valve means responsive to simultaneously open the fluid passage in one back-up plate to admit high pressure fluid between the back-up plates and close the fluid passage in the other back-up plate to prevent high pressure fluid from communicating with the chamber at low pressure.