referring first to fig1 in which a brake band servo is illustrated in a stroked position to take up clearance , a brake band 10 encircling a brake drum ( not shown ) has one end 12 contacting a servo piston stem 14 and the other end 16 contacting an anchor stop 18 fixed in position against displacement on the wall of a transmission casing 20 . the casing contains the components of a gearset , hydraulically actuated clutches and brakes , which control the gearset components to produce the various speed ratios of the transmission . the clutch drum is locked against rotation when the servo piston 22 is forced by hydraulic fluid upward to apply the brake band . the casing defines a cylinder 24 , within which cylinder piston 22 moves in response to the presence of hydraulic pressure below the piston and the effect of the force of a compression spring 26 located between piston 22 and a stop surface 28 formed in the transmission case 20 . a spring retainer 30 includes a flange 32 that seats against stop surface 28 . referring next to fig4 the servo piston 22 carries an elastomeric hydraulic seal 23 , which is sized to fit closely against the walls of cylinder 24 . servo piston stem 14 , fixed to piston 22 so that the stem and piston move as a unit , includes a larger diameter cylindrical portion located at its upper end and a smaller diameter cylindrical portion 34 located at its lower end , these portions forming an annular shoulder 36 disposed in a substantially diametrical plane across the longitudinal axis of the stem 14 where the cylindrical portions meet . an assembly comprising the servo piston 22 and piston stem 14 is arranged with compression spring 26 loosely fitting over the servo stem and spring retainer 30 located above the spring and arranged such that its flange 32 is located at the uppermost end of the retainer . referring now to fig2 and 3 , the lower surface of flange 32 is formed with a diametrically directed slot 40 that passes partially through the thickness of flange 32 . the cylindrical portion 42 includes axially directed slots 44 extending along the length of the cylindrical portion 42 , located at diametrically opposite sides of the cylindrical portion and aligned with slots 40 . retainer 30 has a central opening 46 extending along its length and sized laterally in relation to the diameter of piston stem 14 so that it fits loosely over the surface of the stem . the inner surface of the retainer is formed with a shoulder 48 having a smaller lateral dimension than that of the central opening 46 and the diameter of the piston stem so that the stem produces a slight interference fit causing cylindrical portion 48 to flare slightly outward , which flaring is facilitated by the two axially extending slots 44 . retainer 30 is preferably formed of molded plastic so that as the retainer moves along the surface of piston stem 14 it resiliently grasps that surface and is urged radially inward toward the surface of the stem . when shoulder 48 on the retainer reaches shoulder 36 on the stem , the interference fit between retainer 30 and stem 14 allows shoulder 48 to become engaged on shoulder 36 , thereby preventing movement of the retainer upward on stem 14 but permitting upward movement of the stem relative to the retainer . located immediately below flange 32 is a circular shoulder 50 whose diameter is sized to fit within compression spring 26 so that the upper surface of the spring becomes seated on the lower surface of the flange 32 . an annular chamfer 52 located at the lower side of shoulder 50 facilitates movement of the compression spring onto the surface of shoulder 50 . installation of a brake servo in the transmission casing is described next with reference to fig4 - 6 . the installation begins , as shown in fig4 by placing the servo spring over the piston stem , and then placing retainer 30 on the stem above the spring . these parts are then inserted preferably by hand , into the transmission case so that the stem fits within the pilot bore 54 . piston 22 is then forced upward against spring 26 compressing the spring force being reacted on the stop surface 28 formed in the transmission case . as fig5 shows , stem 14 moves upward into the pilot bore of the transmission casing sufficiently so that shoulder 48 , on the inner surface of retainer 30 , seats and locks on shoulder 36 at the lower end of the piston stem 14 , thereby holding the retainer on the piston stem shoulder against upward displacement relative to the stems and storing the potential energy present in the spring . the lower end 58 of the piston stem protrudes slightly below the lower surface 60 of the transmission casing 20 . thereafter , control gaskets , a plate , and valve bodies 62 are attached by bolting them to the lower surface of casing 20 . this attachment is effected by pushing piston 22 and the piston stem assembly further into cylinder 24 to the installed position shown in fig6 where the control gaskets , plate , and valve body 62 are in contact with the lower surface 60 of the transmission casing . in the installed position , the spring retainer is unloaded because shoulder 36 moves further up the axis of piston stem 14 . shoulder 48 remains in place due to contact of flange 32 on stop surface 28 . retainer 30 is thereafter no longer locked on the piston stem because its shoulder 36 is moved upward out of contact with shoulder 36 . although the form of the invention shown and described here constitutes the preferred embodiment of the invention . it is not intended to illustrate all possible forms of the invention . words used here are words of description rather of limitation ; various changes in the form of the invention may be made without departing from the spirit and scope of the invention as disclosed .