Train encountered uncommanded braking during departing SBW2. TC reported TIMS did not show any fault . Fault recurred departing KTB2. STBS was operated. ~~~~~ L3   Downloaded ER PTU006. From ER observed BRER dropped during 1423hrs to 1425hrs. During brake cyclic test found #1683 ACU air leaking from exhaust port. Residual pressure of 0.3bar, causing BRER to drop. Replaced ACU and carried out brakes cyclic/function test ok. Checked brake command line-ok. ACU-Old s/n: 1549-212.  New s/n: 1937-233. Not: 2153433 raised to RSW. Done by Kumar/Jack/Sateesh/Koh. 23/04/20.