TC reported train came to a gradual stopwith stuck brake on rear DT car. STBS was operated and TC p roceeded in ATPM to BSH1 HW to disembark pax and withdraw toBSD. ~~~~~ #3553 ER & fault trace downloaded on PTU006. Checked found #3553 ACU ac s/v heavy air leak when coast to B3.    Replaced ACU and carried out brakes cyclic/function test ok. ACU-Old s/n: 1250-052  New s/n: 1039-022. Not: 2137570 raise to RSW. Done by Jack/Sateesh/Koh  04/03/20.