First and second hydraulic servo units operate respective frictional engagement elements in a transmission and each includes first and second cylinders, each having a side wall portion interposed between the two frictional engagement elements. The first cylinder has first outer and inner cylinder portions and the first hydraulic servo unit further includes a first piston having inner and outer cylindrical surfaces slidably supported by the first outer and first inner cylinder portions of the first cylinder for reciprocating axial motion into and out of engagement with one frictional engagement element. First return springs are arranged radially inward of the one frictional engagement element for returning the first piston. The second cylinder includes second outer and inner cylinder portions and the second hydraulic servo unit further includes a second piston having a larger diameter portion and a smaller diameter portion, slidably supported by the inner cylindrical surfaces of the second outer cylinder portion and the first inner cylinder portion, for reciprocating axial motion into and out of engagement with the second frictional engagement element. Second return springs are arranged around the inner circumference of the one frictional engagement element for returning the second piston.