Raoul,
Is it shiny red or shiny ally with red label?
The filter can be a pain to remove as the heater return hose has to come off first. I would warm up the car first and remove the hose to the csv from the inj pump and block it with a 12mm bolt. Then fit another hose to the inj pump adaptor, start the engine and see if you can suck/blow thru that hose.
naj
'Kloines Scheisserle'
65 230SL
68 280SL