[Capt. Debbie]

The filler behind the steering wheel?

The reservoir keeps fluid above the moving parts and keeps them immersed under the hydro fluid surface. Under the surface no air can get in the "works".

So no air enters the steering ram system since it is under the reservoir surface by design. If that reservoir drops down and the surface falls below exposing the "works" you get air in the steering system and need bleeding out of the steering lines going to the ram moving the O/B. Hence you feel bumps and pulse feeling when you turn the wheel with air in the steering system.