What was found
PHX - AIR TURNBACK - FLT 27 - AFTER T/O THE LANDING GEAR WOULD NOT RETRACT. CREW RESET LGCIU 1 & 2 PER MAINTENANCE CONTROL, NO HELP. THE AIRCRAFT APPEARED TO REMAIN IN GROUND MODE. THE AIRCRAFT RETURNED TO PHX, MAKING AN UNEVENTFUL LANDING. MAINTENANCE TROUBLESHOT AND FOUND THE PRESSURE IN THE UPPER CHAMBER OF THE LT MLG STRUT TO BE 480 PSI AND LOWER CHAMBER AT 1250 PSI. MAINTENANCE REPLACED THE UPPER SERVICING SCHRADER VALVE, SERVICED THE STRUT, LEAK CHECKED THE VALVE AND PERFORMED GEAR SWING, ALL FUNCTIONAL CHECKS NORMAL. NO FURTHER FAULTS REPORTED.