What was found
(CAN) DEPARTED ENROUTE 30 MIN AFT. DEPARTURE CLIMBING THROUGH FL310 POP/BANG HEARD FOLLOWED BY HYD LIGHT THEN LT HYD PUMP LIGHT THEN DECREASE IN HYD PRESSURE TO 1500 PSI. RT PUMP LIGHT ON PRESSURE DROP TO 1500 PSI. A/C TURNED AROUND FOR YYZ, FUEL DUMPED, AS THEY DESCENDED BELOW FL250 HYD SYSTEM RETURNED TO NORMAL NO LIGHTS. LANDED GROUND CHECKED FOUND HYD CAP OFF NR 1 SYSTEM WITH NO FLUID LOSS. CAP CHECKED FOR DAMAGE NONE FOUND CAP REINSTALLED A/C GROUND RUN SYSTEM CHECKED OK AND RELEASED TO SERVICE.