is that side low all the time or just when the car has been shut off for a while? if it's all the time then chances are that it's the valves, if it occurs after the car has been sitting i'd suspect a leak somewhere.
the suspension in the 6.3 is notorious for leaking. the leaks can be hard to find and expensive to fix.
'94 W124.036 249/040 leder; 8.25x17 EvoIIs
'93 W124.036 199/040 leder; 8.25x17 EvoIIs, up in flames...LITERALLY!
'93 W124.036 481/040 leder; euro delivery; 8.25x17 EvoIIs
'88 R107.048 441/409 leder; Euro lights
'87 W201.034 199/040 leder; Euro lights; EvoII brakes; 8x16 EvoIs - soon: 500E rear brakes
'70 R113.044 050/526; factory alloys; Euro lights