What was found
PHX\JFK - FLT 290 - AIR TURNBACK, DURING CLIMB-OUT, CREW REPORTED EICAS 'L HYD QTY' MESSAGE. CREW VERIFIED QTY WAS DROPPING QUICKLY WITH DECREASING PRESSURE. DURING RETURN TO PHX, CREW REPORTED EICAS 'POWER XFER UNIT' MESSAGE. CREW DECLARED EMERGENCY, LANDING WITHOUT FURTHER INCIDENT. AIRCRAFT WAS TOWED FROM RUNWAY TO GATE. MAINTENANCE REPLACED HIGH PRESSURE DOWNLOCK ACTUATOR LINE AT LT MLG. LEAK CHECK COMPLIED WITH. AIRCRAFT RELEASED FOR SERVICE. (X)