The plastic piece actually is the marker for the fluid level. If you unscrew the nut (be careful it is spring loaded), you can change the p.s. filter, it just lifts out with needle nose pliers, and is only $6ish. On the early ninety MBZ cars the level a couple on inches below the top, probably NOT a good idea to overfill. I do NOT want to get into a discussion of what to refill it with

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1994 E320 Cabriolet
1992 300CE (Sold)
1991 300SE (Sold)
1986 560SEL (Sold)