You're correct that the return line does not have much pressure, it's all in the input line to the steering gear assembly.
Filter? Nope, I'd not recommend that for the power steering line. IMHO you're better off changing the fluid once a year and puting the power steering assembly on a good flush/fill/bleed maintance schedule. You'll be getting most of the gunk out anyway if you flush and re-fill when you re-assemble the power steering. This decision would, of course, be affected by where and how you're driving your rig.
If you are intent on making the Power Steering System better, and helping it to last longer I'd recommend three low-buck items:
1. Don't just clean up that steering box, actually rebuild it (or the old one and put on the shefl for a spare [img]images/icons/smile.gif[/img]) with a seal and gasket kit from NAPA.
2. Rebuild the power steering pump. These kits are also avaliable at NAPA.
3. Install a power steering fluid cooler in the input line. Some people think this is overkill. But if you're in a hot area then it will help save seals in the gear box and in the pump assembly. The cost is low, and the payback is high.
You may be interested in viewing a Cooler/Filter install on a Bronco... a Jeep install wouldn't be much different, click on the picture. Heck, I think you'd even make it look prettier!
Just be aware that filters and coolers will add another point of faiure to the system as longer hoses can flex, split, burst, degrage as well as the hose clamps can fail.
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