TM  10-3930-638-24&P 2-43. SERVICE BRAKE MAINTENANCE (cont) c.  Hydraulic Brake Valve (cont). (2)  Power  Assist  Unit  (cont). 13 STEP LOCATION ITEM ACTION REMARKS REASSEMBLY/REPAIR   (cont) d. U-cup seal (27) Install In groove of spool cap bore, with lips of (cont) and O-ring (26) seal (27) facing open end of bore e. Spool (9) Position  and In soft-jawed vise, with narrow end of clamp spool facing up f. Spring (25) Install Slide over narrow end of spool (9) CAUTION Exercise care when installing spool cap (11) so as not to damage U-cup  seal  (27). g. Spool cap (11) Install Push wide end of spool cap (11) down over spool (9) and against pressure of spring (25) h. Retaining ring (24) Install Push into groove at end of spool cap, and close with pliers until ring is tight in groove 14 Gland (2) a. O-ring (21) Install In U-cup seal (22) b. Gland (2) Lubricate Lightly coat bore with hydraulic oil c. Backup ring (23) Install Push against flat side of seal (22) in gland bore d. U-cup seal (22) and Install In groove of gland bore, with lips of seal O-ring (21) (22) toward inner face of gland e. O-ring (20) Install In groove at outside diameter of gland (2) 15 Power  assist a. Body (5) Position  and In vise, with gland end facing down unit clamp b.  Poppet  (19) Lubricate Lightly coat with hydraulic oil CAUTION Exercise care so as not to scratch or damage poppet (19) during installation. c. Poppet (19) Install In relief valve bore of body (5), with narrow end of poppet facing up. Be sure poppet goes all the way down to the seat NOTE Spring (17) and shim(s) (18) are matched for proper relief pres- sure. Reassemble power assist unit using origrinal spring and shim(s)  only. 2-307