Item K in the drawing is the fill-plug. IIRC, it's a dense rubber plug. IIRC - I think some folks would use power steering fluid, others transmission fluid, but you don't want to mix them by topping off with something different than what is in it.
For me, it was all new parts, hoses and the side hydraulic arms as well so I used ATF. I think instructions noted to work the top up and down a few times to get out all air bubbles, then fill to that sight hole, fill hole.
I believe GM used brake fluid (they did in '63; a year I'm much more familiar with). Brake fluid works just fine but it is rather corrosive/damaging if spilled on painted surfaces (nothing a quick thorough clean up won't handle).
IF you are simply adding fluid, don't mix. If it has brake fluid (it has a unique smell); add brake fluid. Pack a bunch of rags around the pump to catch any spillage. Fill to just below the fill opening. Don't close up yet. Cycle the top up/down a few times to remove any air. Check level; add fluid; repeat. Close up.
If you still have problem, you may have a leak in the system somewhere.
When I bought my 65, the top worked perfect. When the owner took it for the safety before I picked it up, the top motor quit. He had a garage swap it with a spare he kept from his old 65 SS. Garage did the swap, they filled with ATF. Still works perfectly 25 years later.
For your issue, I'd disconnect the cylinders and do some trouble shooting first. Pump's working one way, should work the other way as well. FSM likely has a trouble tree to follow.