99 Tahoe Power Steering Issue

[londo]

just replaced the pump[rebuild kit] andsupply/return lines.Flushed/bleed/ system per instructions. All good until.. Turning into the driveway [while applying brakes] damn power steering goes out[hard to turn]. Park turn wheel left/right all the way ..no problem. Thinking maybe air bubble or something. Wife drives it, same thing. I drive around for a couple of days, nothing. Turning into parking stall today, goes out. Come home into the driveway, goes out. I know it's wierd but always turning right when this happens. Last time i did one was on a late 60's 350 and I swear the pump is the same except for the sensor on the back of the pump at the dogbone line connection. What is this thing and what does it do ? I am stumped ! no leaks, no ideas !