B737-800 Technician reported omitting a step that required a leak check of the left brake metering valve. The aircraft experienced the loss of system A hydraulic fluid and performed an air turn back and landing at departure airport.

Narrative

Aircraft X had a maintenance set up to replace the left brake metering valve. I picked up from midnight shift with two hydraulic lines to install. I was the fourth AMT to work the valve replacement. B737 AMM step G (1)Leak check; (2)Gear retract braking actuator check; (4) Bleed the hydraulic systems. I installed the 2 lines; restored head pressure; and began the check out step G(1). Was working alone so after turning on A/B hydraulics was running up and down to make sure no leaks were present. No leaks; shut down and secured the aircraft. Went to work another AOS (Aircraft Out Of Service) aircraft. When I came back to Aircraft X; I continued the checkout step G(4) with the assistance of my Lead AMT; no leaks; I signed the logbook and returned to service. When Aircraft X took off; at gear up they lost A hydraulic fluid and came back. Step G(2) is also a leak check G(2)(e). Had I not missed that step I'm sure we would not have had an event. The leaking line had not even been tightened and looks like it was removed to assist the valve removal.