As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK/DECLARED EMERGENCY. ON TAKEOFF, AFTER RAISING GEAR MASTER CAUTION, FLIGHT CONTINUED AND HYDRAULIC LIGHTS CAME ON. SHORTLY AFTER F/A CALLED THAT CABIN WAS FILLING WITH BAD SMELL AND FOGGY/SMOKE. REMOVED AND REPLACED HYDRAULIC RESERVOIR PRESSURIZATION RESTRICTOR CHECK VALVES PER MM.