As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT NR 3159 - EN ROUTE, THE AIRCRAFT LOST HYDRAULIC PRESSURE ON THE LEFT SIDE. THE NR 1 STANDBY PUMP WAS IN THE NORMAL POSITION. THE CREW PERFORMED QUICK REFERENCE PROCEDURES AND RETURNED TO DTW. MAINTENANCE REPLACED THE LEFT HYDRAULIC PUMP. ALL CHECKS SATISFACTORY. (M)