TC reported that MR pressure at 8.8 bar.No signs of charging up. Subsequently, MR pressure dropped to 8.4 bar. Passenger disembarkation was initiated at BGS WB . Train was sent to OTP siding. ~~~~~ 3605 Reported MR dropped: Level 3 (NSEWL) T127 T127(3605) MR dropped to 8.6 bar in between LVR-BGS2. Unable to charge up. Train withdraw service at BGS2 as precautious measure. Train route to OTP SD. EMU 3605 TC reported that MR pressure dropped to 8.8 bar with no signs of charging up. No fault indication. Subsequently, MR pressure droppedto 8.4 bar.    Pax were disembarked at BGS WB platform. After disembarkation, TC reported MR pressure charged up to 10.4 bar.Train was routed to OTP SD. At TPG WB, TC reported MR pressure dropped to 8.6 bar. No air discharge sound was heard. At OTP SD, TC changed end to East cab and feedback no air discharge sound was heard. MR pressure on East cab was 10.1 bar. A scheduled withdrawal T326 ahead was used as a replacement train. L3 605/606 checked side & U/F all air hoses/pipings for any abnormal air leakage - OK. Checked brake status & brake units for air leakage - OK. Checked/found #3605 air compressor cut-in at 8.28 bar, cut-out 10.43 bar. #3606 cut-in at 8.36 bar, cut-out 10.22 bar. Found #2605 ACU abnormal noise. Replaced #2605 ACU & tested - OK. OPS 20 raised to replace #3605 & #3606 CG when spares are available.  Chan GF/Alan 44449 #2605 ACU S/N : OLD : 1442/034 NEW : 1403/006 To follow up rectify. L3 605/606 fault unable to be duplicated in. ER showed train was in AM when TC reported fault. NO related FC observed & Line Voltage was Normal on all cars. Interviewed TC & feedback that MR pressure at #3605 was dropping from 10.4 bar to 8.6 bar within 2 mins when train was in service (leaving/entering station). Similar abnormalities was also observed for a few more stations. Air compressors checked & able to cut-in/out. Air dryers able to purge & switch over. Dump v/v function test - OK. CCD shoes/fuses checked - OK. CK & CKAK contact resistance checked for both DT cars - OK. CPTR1 delay start timer checked - OK. FINDINGS : Found #3605 CG cut-in at 8.28 bar & #3606 CG cut-in at 8.36 bar, Nominal is 8.5 bar. No unusual triggering from ATC Brake command when train was entering/leaving station. Signal LDC log showed no abnormal command. W/S/S not active across all at time of incident. Both compressors function check, able to cut-in/out - OK. LMRG functioning within spec & no trigger in ER. RECTIFICATION : Replaced both DT car compressor governors (CG). Air Compressor function check : #3605 : Cut-in (8.69 bar), Cut-out (10.39 bar),         Charging time (2m 04s) #36.6 : Cut-in (8.58 bar), Cut-out (10.38 bar),         Charging time (2m 08s) Air leak test : 0.27 bar/5 mins Air dryer change-over - OK. SIDE FINDINGS : Found #1605 & #2605 ACU intermittent hunting.    Replaced & function chec - OK.  Toh CH 44451