I simply cleaned out the old filter, but I did replace the rubber gasket around the metal flap, which had become very brittle (watch out if you glue it on: it goes on one way and the first time I did this, I put it on backwards). I ran drops of household oil down the cable than controls the flap, holding it up. Same method to grease brake cables of motorcycles, bicycles etc. The flap pivots on two points, located on either side, with plastic washers. I put some lithium grease on those. The flap now opens and closes quite lightly.
When the round air vents inside the cabin are closed, there should be NO air coming through. If there is, you can adjust the ball linkages that are located just behind the air vent louvers. Good idea to clean and grease those, while you're at it.
Cees,
white 1969 280 SL