Possible Leak Issue on Older W126
 

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.

Washer Tank leaks
 

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.

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.