What was found
(CAN) FLIGHT DEPARTED AND ACFT BEHIND WITNESSED VAPOUR COMING FROM RT ENGINE. ONCE IN THE AIR HYDRAULIC PRESSURE IN RT PRIMARY PUMP WENT TO ZERO. MAINTENANCE ADVISED AFTER LANDING, IT WAS A BLOWN SEAL. CREW DECLARED AN EMERGENCY WITH ATC AND AFTER QUICK CONSULT WITH DUTY PILOT ELECTED TO DUMP FUEL. FLIGHT ORBITED AT 10000 FT FOR 30 MINS AND DUMPED 30,000 KGS, FUEL DUMP MESSAGE COMPLETED. FLIGHT RETURNED WITH A LANDING WEIGHT OF APPROXIMATELY 287,000KGS, APPROX 55,000KGS, OVER MLW. EMERGENCY SERVICES WERE ON STANDBY ON ARRIVAL TO INSPECT BRAKES RESOLUTION: RT EDP REPLACED IAW AMM 29-11-05-02B.