507/508 service brake application due to ~~~~~ To further rectify.  1507 raised Notif 2218364 to RSW due to #1507 A3 parking brake jammed.  Au TK    44146 507/508 service brake application due to undershoot at station. Refering to Notif by SIG; 2218261. To conduct inspection on  1507 ER D/L & BCP check - OK. No air leak on all cars BCU. No Fault codes recorded. Physically checked on all brake units & found #1507 A3 parking brake unit jammed, unable to release when DMS @ COAST. Notif 2218364 raised to RSW. BC2 isolated/tagged. Name                          : Au Tuck Kim Date of Occurrence            : 11/11/2020 Last Service Date             :   EMU / Equipment / Location    : 1507/Bogie 2/A3 Parking Braking                                  Unit Equipment Serial No. (if any) :   Description of fault / cause  : Unable to retract when DMS put                                   to COAST. Isolate BC2 cock                                       (tagged )and manually retract                                    parking brake unit. Caused                                       train to be under-run at                                         mainline.  Au TK 44146