What was found
CREW ADVISED CAS ILLUMINATED. FLAPS & GEAR BOTH RETRACTED. FOLLOWED CHECKLIST & USED EMERGENCY BRAKING, BLOWING BOTH LT MAIN TIRES. CAS NOT ASSOCIATED WITH GEAR EXTENSION, & EMERGENCY BRAKING PROCEDURES HAD TO BE USED IAW CHECKLIST. MX REPORTED ON ENGINE RUN, ACFT HAD NO BRAKING ACTION WHEN APPLIED. MX REPLACED BRAKE CONTROL VALVE & SWAPPED ANTI SKID VALVES FOR FURTHER TROUBLESHOOTING. OPS TESTS CONDUCTED NORMALLY. ON NEXT ATTEMPTED TRIP, PILOT REPORTED THAT AS SOON AS THEY RELEASED PARKING BRAKE TO TAXI OUT, SAME CAS MESSAGE ILLUMINATED. TRIED TO POWER DOWN & START BACKUP BUT MESSAGE STILL POSTED. AFTER ADDITIONAL TROUBLESHOOTING, FOUND PIN A ON MT57 PUSHED BACK. REST PIN & RECONNECTED. OPS CHECKS NORMAL. TAXI TESTS NORMAL. ACFT RETURNED TO SERVICE.