Over the past 60 years or so, steering assist systems have been built with hydraulic, electric, air, and even vacuum as a power source. Hydraulic systems have been the odds-on favorite since it's fairly simple to run a belt-driven pump and valve the boost pressure to either end of a cylinder to make the driver's turning effort less. The main gripe has been the amount of power that gets robbed from small-displacement engines. The self-contained power system incorporated into a steering gear such as the GM Saginaw unit has become the method of choice for most manufacturers. It has a spool valve that directs pressure to either end of an internal piston, all contained inside a fairly compact unit that's not prone to leakage or failure for 100K miles or more. I believe there are much better ways to adapt that type of system to a stovebolt straight axle than what's currently available. The ram-assist style power system is much more prone to leakage and/or damage to hoses. For a hobby-type vehicle that doesn't get a lot of mileage, wear is sort of a moot point compared to a daily driver. Some of the air-assist systems I've worked on for over the road trucks are really interesting!
Jerry
