As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FROM KMLB, THE CREW NOTICE THE LEFT PNEUMATIC MANIFOLD READ'S 17 PSI LOWER THAN THE RIGHT SIDE, THE LEFT SIDE READ'S 20 PSI AND THE RIGHT SIDE READ'S 37 PSI. THE LEFT SIDE MANIFOLD COULD NOT REACH 35 PSI TO DO PRESSURE DECAY CHECK, THE LEFT SIDE ONLY WENT TO 28 PSI AT POWER. THE CREW RETURNED TO KMLB, AND LANDED WITHOUT INCIDENT. MAINTENANCE REMOVED AND REPLACED NR 1 AND NR 2 PNEUMATIC REGULATOR VALVE IAW DACO DC-8 MAINTENANCE MANUAL 36-12-1. OPERATIONAL CHECK WAS GOOD. AIRCRAFT WAS RETURNED TO SERVICE.