What was found
A/C 743 WAS IN CRUISE WHEN 'B' SYSTEM LOW QUANTITY LIGHT CAME ON FOLLOWED BY GAUGE GOING TO ZERO. 'B' SYSTEM LOW PRESSURE LIGHTS ILLUMINATED, BOTH 'B' PUMPS SELECTED OFF. OVERHEAT LIGHTS DID NOT ILLUMINATE. A/C WAS EN ROUTE TO YEG, ATC WS ADVISED AND EMERGENCY VEHICLES WERE REQUESTED TO BE ON STAND BY. THE LANDING WAS UNEVENTFUL AND THE A/C TAXIED TO THE GATE. MAINTENANCE CONFIRMED THE SOURCE OF THE LEAK WAS THE 'B' SYSTEM PCU. THE PCU WAS REPLACED AND THE AIRCRAFT RETURNED TO SERVICE WITH NO FURTHER DEFECTS RELATED TO THE HYDRAULIC SYSTEMS TO DATE.