The reservoir is where excess fluid is stored. You should have a pressure guage and a air valve on top. Bleed off the air pressure and remove the large plug. Fill the resrvoir using a funnel, install plug and pump up the pressure to 20 -25 psi. Your helms will push out the air by rotating back and forth with full cycles. Start at the highest helm. You may have to continue to add fluid. If you continue to loose fluid, then check for leaks, at steering ram, the helms, any connections to A/P or whatever you can find. New steering systems do not use the reservoir, fluid is stored in the helm.
Bob