From my other postings (seems like a lot on A/C systems today) I would replace all the systems hoses as long as you have the system apart. Also I would change over to R134a. You do have to have a rotary compressor but your '80 240D should have that. Ther are other things that need to be done and flushing the system is the MAIN one (not cheap).

Just my opinion on this from "rebuilding" the climate control in a '79 300D and converting it to R134a in the process. (total cost was over $1000)

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current Benz
'87 300TD (white) 168K (on the odometer as of 15 April 2004)
past Benz owned & miles I put on them
'62 190D 20K (white)
'63 190D 20K (beige)
'65 190D 35K (white)
'73 220D 175K (metalic green)
'79 300D 135K (metalic blue)

driving a Benz diesel since'77