1969 Autocar (I was told at least)

Air assist was an add on and could be on any truck with a manual box. The three 1984 4300 IHC's I converted to hydraulic power steering had those switches. If your Mack was older it may have been different.

 

Later models without the flat dash probably had the breakers in the kick panel on the left side of the clutch pedal.

 

I never cared for their clutch linkage setup. Say what you will about the Peterbilt clutch through the floor, but on mine, it was the nicest clutch I ever used, when I used it, that is. Those White products were great trucks. I made money with my Western Star, because worked every day.

 

I have never seen a system like that on a truck. Once worked at a place with two cranes made in the 60's. One was a P and H, the other a Manitowoc. One or both of them had a similar system. Never had drive ability problems as they never left the sight.
Yes that is a hydraulic assist steering system and the cylinder with multiple hoses is the control valve.
I believe it would have some of the same struggles as the air assisted system. If there is anything that would cause the front end to wander or or dart you can be in for a constant struggle.
Of the three 8400.s mentioned one driver was fine with it and left the air switch turned on all the time. He also did not have to back into tight spots. He explained that if you know how the system works and do not make abrupt changes with the steering wheel it drives fine. However if you jerk the wheel the control cylinder sends air to the steering cylinder. That causes over steering and you jerk to the other way. It can be the beginning of the tail is wagging the dog situation. Try to keep the steering wheel straight in the direction you want to go and make slow corrections.
Check alignment plus everything for wear front and rear.
After that you may need to rebuild the control and slave cylinders. Hydraulic packing does not last for ever.