TC reported rear M2 brake fault upon departing from station. STBS was operated and TC ATPM to next station. TC overshoot next station and was authorised to cra wlback. Pax was disembark and SM(YCK) isolated bic at rear M ~~~~~ Check and found #2667 & #3668 ACU heavy air leak from solenoid v/v exhaust port. Replaced and brake function test ok. Both cab STBS re-seal back. #3668-ACU-Old s/n:1535-066  New s/n:1535-096 Not: 2125285 #2667-ACU-Old s/n:1511-085  New s/n:1538-167 Not: 2125246 Done by Jack/Koh. 29/01/20.