2000 Silverado Steering Problem

I'm having a problem with hard steering in one direction only. Mostly at slow/no speed. Lost some fluid out of the system a few weeks ago when the problem first occurred. Refilled resevoir and has remained full although still difficult turning one direction (left). I also hear a hissing sound through the steering column when turning left.

2000 Silverado 1500, 5.3L, 140k mi, steering gear (not rack).

Does this sound like problem with steering gear, pump? Any suggestions appreciated.