The control valve has three different valves inside.
1) Base pressure valve, which maintains base pressure to set correct ride height when vehicle is unloaded.
2) Adjusting valve to increase beyond base pressure when loaded (this is the one connected to the arm/rod/torsion bar).
3) Overpressure valve (safety relief).

If you leave it in the 'Fill' position and keep a higher-than-normal pressure in the system, the base pressure valve WILL fail, and when you park the car the rear end will sag. Eventually, it will sag all the time.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC