Hi
The steering purge should be done by the operator regularly, if it has not been done for 3 weeks that is an operator issue. If not done this would also increase tyre wear. Assuming there is nothing mechanical wrong, a purge may need be daily or a couple of times per day if the truck is running mostly long distances with long loads in sideways mode all day. There should be a decal in the cabin explaining the way it has been explained below. The explaination is also in the manual. YouTube Video link https://www.youtube.com/watch?v=g5F_g_tK75s

Hope this helps,
Richard