Glad your pumps aren't leaking anymore after removing that part from the cap vent. Can't find in a search if on a correct vented cap if it's 1-way or 2-way but in your test it sounds like it's 1-way and not supposed to be letting air in.
The last step on the "Pump Leakage Diagnosis" from the 96 FSM (p. 19-5) states, "6. Check oil level; if leakage persists with the level correct and cap tight, replace the cap.". So maybe if it lets air in it also lets fluid out and there's supposed to be a slight vacuum. Kind of like if you hold a straw full of liquid with your thumb on the top it won't let any of the liquid drip out but once you release your thumb it all comes out.
Since you're saying your steering turns easier I may have to try something like you did, unplug the vent, or get a new cap. Ever since I put my last rebuilt pump on it's seemed to steer hard. Figured maybe it had the wrong high pressure line fitting w/flow control orifice. I remember a thread about the number of notches/indents on the nut sides are supposed to tell which flow control orifice it is. Venting a cap sounds like a whole lot easier solution!