29PSI standing pressure means the system is just about empty. You're probably locked out on the low pressure switch to keep the compressor from engaging. I'd suggest finding the leak before recharging. If you wish to skip the leak search and just dump refrigerant in it, you can charge through the low port even though the compressor doesn't run. With the A/C off, charge a can as a liquid through the low side port. You should be able to get most of the can in there (it'll turn to gas as it enters the system) and should raise the pressure high enough to engage the compressor so you can finish charging.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]