What was found
DURING APPROACH TO MBS WHEN SLATS WERE EXTENDED, THE AIRCRAFT ROLLED SLIGHTLY TO THE LEFT. SLAT DISAGREEMENT LIGHT ILLUMINATED AND THE BLUE SLAT EXTEND LIGHT DID NOT ILLUMINATE. SLATS AND FLAPS WERE RETRACTED, FOLLOWED COM PROCEDURES, AND RETURNED TO DTW AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE LEFT SLAT HANGING UP. ALL CABLES AND ACTUATORS CHECKED OK. REPLACEDTHE SLAT CONTROL VALVE AND LUBED THE 'T' DUCT AND ROLLERS. OPERATIONAL CHECK OK. AIRCRAFT RETURNED TO SERVICE.