What was found
SEA- ON FLIGHT NUMBER 2008, RETURNED TO FIELD ON TAKEOFF, MASTER CAUTION RUD LIMIT FULL PRESSURE. UNABLE TO RESET MASTER CAUTION. CAUTION LIGHT WENT DIM TO BRIGHT THEN BACK TO DIM. HYDRAULIC PRESSURE AND QUANTITY NORMAL. REMOVED AND REPLACED HYDRAULIC PRESSURE SWITCH AT P3-P4. RAN ENGINES TO PRESSURE SYSTEM. OPERATED RUD LEFT/RIGHT TO OPERATIONAL CHECK SWITCHES WITH FLAP AT 0 & 5 DEGREES. LIGHTS ILLUMINATED, RESET MASTER CAUTION, RESET RUD PRESSURE. AIRCRAFT RETURNED TO SERVICE.