My car started running low on washer fluid all the time. I did a closer inspection and found a rubber plug that was partially disintegrated and leaking. It's at the bottom of an indentation of the plastic tank where another pump motor might go. It's easy enough to see by shining a light down the back of the tank. I replaced mine with a screw and washer.

A slow leak like that can further aggravate the rust problem in that area.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

Yes, I just had that problem on my 89 420SEL ... the rubber grommets that fit into the washer pump holes disintegrated. You can get those from Phil or the MB dealer and insert them, putting the pump down through them into the tank. The second hole is for the headlight washer system that is on 420's and other models of that era and of course a second grommet and pump goes into that area.

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Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)

Yep, I have replaced all the tubing for the washer system. It was all in poor shape and leaking. Running the tubing for the headlight washers was a bit of a pita.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200