You have to bleed the box.
1. While holding the adjusting screw with a flat blade screw loosen the jam nut about 4-5 complete turns.
2. Make sure the washer is up against the jam nut, be careful because there is a O-ring on the center of the washer. You do not want to tear it or you will be looking for a source for said washer.
3. Start it up, turning the wheel back and forth watching the jam nut until no air leaks out, only fluid.
4. Holding the stud with a screwdriver tighten the jam nut.
Some of the boxes have a bleeder like a brake, slip a hose on it, stars it up, loosen the bleeder and turn the wheel back and forth until no more air comes out.
Use only ATF in the system, not power steering fluid.
This is what the washer looks like.