TC reported Rear M1 BCU fault. Fault self cleared. ~~~~~ #1690 found air leak from exhaust port when 750vdc cut/out. Replaced ACU and brake cyclic & function test-ok. #1690-ACU: Old s/n: 1549-214.  New s/n: 1523-120. Not: 2123055 raise to RSW. Done by Jack/Koh.  21/01/20.