Heres a question for you "old-timers" out there:
My 1977 6.9 settles (sinks) over time due to, probably, leaky leveling valves. Assuming that bad valves are causing the sinking, then the problem could be either with the front or rear valves individually or with the pressure regulator valve. The car sinks both in the front and the rear, but noticibly more in the front.
After reading through the very-hard-to-get service manuals, I devised an experiment. Since the car sinks with the control valve at the "normal" (height) setting, what would happen if I set the valves to the "locked" setting (i.e., the setting required when towing the car, etc.)? I set the valves to the locked setting and, lo and behold, the car did not sink (and has been sitting this way for a couple of weeks now).
So, you 6.9 super sleuths, what did my experiment tell me? Is my sinking problem with my front and/or rear leveling valves? Or is the problem the pressure regulator valve assembly? Or none of the above? I'm just trying to avoid replacing all of the valves in an attempt to solve this problem. (Could it be just dirt in the lines?)