If there's air in the system they generally need to be bled by opening the nipple on top of the steering box, otherwise the box can develop an air lock inside.
To do this, start the engine and turn to full LH lock and let go of the steering wheel. Bleed until clean fluid comes out, then do the same on the other side.
Do not hold at full lock for more than a second or two otherwise you can damage the pump.
This will bleed and prime the system.
Catch any fluid in a clean container as it could have air suspended in it.