Undershoot more the 3mTrain undershoot 3x at TAP2 time of fault ( Kindly checked BCP ~~~~~ 3550 Undershoot more the 3m. Train undershoot 3x at TAP2 time of fault (1706,1939,2211). Kindly checked BCP.  3550 ER D/L & found #3550 brake pressure was slightly higher at the 3 reported time of fault. Replaced ACU & brake cyclic test - Satis.    Brake shoes condition checked - OK.  Nelson/Kana/Sarma 44507 #3550 ACU S/N : OLD : 1024/005 NEW : 1302/050