What was found
AIR TURNBACK, AFTER TAKEOFF DURING FLAP AND GEAR RETRACTION YAW DAMPER, FEEL DIFFERENTIAL AND FLIGHT CONTROL BELOW PRESSURE LIGHTS ILLUMINATED. WENT TO STANDBY RUDDER -B- AND LIGHTS EXTINGUISHED. AIRCRAFT RETURNED TO IAH AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND SYSTEM -A- RESERVOIR OVERSERVICED. SERVICED TO CORRECT LEVEL. PERFORMED OPERATIONAL CHECK ON LOW PSI WARNING PER BOEING 737-300 AMM 27-31-00, PERFORMED ELEVATOR AND TAB CONTROL SYSTEM TEST IAW BOEING 737-300 AMM 27-31-00, PERFORMED HYDRAULIC MODULAR TEST IAW BOEING 737-300 AMM 27-09-21 PERFORMED LANDING GEAR TRANSFER VALVE OPERATIONAL TEST IAW BOEING 737-300 AMM 32-31-71, PERFORMED POWER-ON HYD LEAK CHECK, ALL SYS CHECKED NORMAL.