Make sure you don't still have a leak and your pressure is just too low to activate low pressure switch now. '87 300D should take ~34oz. of R134a. If you're using 12oz. cans (and only two) you're way short, also. Definitely replace receiver/dryer,(system flush would be good) (new expansion valve for 134a is good; although on MB they are probably the same as the R12 valve to be honest). Add correct amount of oil (PAG or ester) (about 4.1 oz. in a flushed system for '87 to "89 MB) and pull a good long vacuum. Charge into the vacuum with the first can (either as liquid or gas) then add second can while checking pressures, then SLOWLY add third can while really keeping an eye on pressures, particularly high side (2.2 to 2.5 times ambient temp. in *F).
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)