What was found
HYDRAULIC REGULATOR WAS INSTALLED IN THIS AIRCRAFT AS A OVERHAULED, EXCHANGED. THIS UNIT FAILED DURING TAKEOFF, WHICH PREVENTED HYDRAULIC PRESSURE TO BUILD UP TO SYSTEM PRESSURE. UNIT WAS REMOVED AND REPLACED BY THE OWNERS ORIGINAL REGULATOR AFTER IT WAS OVERHAULED BY AN OUTSIDE VENDOR. OPERATIONAL CHECK OF LANDING GEAR PERFORMED, ALL FUNCTIONS NORMAL. FAILED UNIT WAS RETURNED TO VENDOR FOR WARRANTY CONSIDERATION.