Can someone tell me what this is? It was leaking, I opened it up and it was just empty except for oil. Thanks. Obviously I am a novice.

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I cannot answer your question. Sorry.

But is there a chance you actually have a 240d there? I only see evidence of 4 cylinders.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

I should clarify. The picture is not of my 300D. I used a pic I found online while searching for a diagram of the engine parts. I tried to add a red circle to the picture on the part in question. Sorry I was unclear, it was late. Thanks.

Power steering reservoir. Fill with ATF.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Thanks Mike,

Do you know if there is supposed to be a filter of some sort in there? The car is new to me so I'm not sure what type of fluid is currently in there. Would I have to drain it before adding the ATF? I need to find some sort of washer to try and stop the leak, currently there is just a wingnut holding the lid in place without any washer.

If it's leaking from the top, could be overfilled

The better fluid to use is Febi Power Steering Fluid which you can buy from this site. There is a filter at the bottom of the canister. Make sure you don't buy the early version. The later version is flatter and has a metal casing. The early one just looks like a round, un-cased filter. I may have a pic if you need.
You can unplug the fluid return line at the canister and run it into some sort of container while filling the main reservoir with new fluid and flushing the system out. Do this with the front wheels off the ground, engine running, turning the wheel slowly left to right. DO NOT LET THE reservoir run dry. You will need an assistant.
The other easy way is to just turkey baster out as much as you can out of the res and add fluid. Turn the wheels, mix up the stuff, baste out some more. Keep going until you feel satisfied with the cleanliness.

Of course, this is after you fix the leak.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

There didn't seem to be any filter in there when I opened it up, could you post a picture or a link to the part on Pelican? I have also seen a gasket for the reservoir cap for sale but it says it was for the 123 chassis. Is there one for the w115 or is the reservoir the same in both models? The reservoir was about half full with a gold colored fluid that seemed pretty clean. I am hoping by adding a washer under the wing nut and possibly adding a gasket under the cap, I can stop the leak. The car seems to have a few leaks and I am just trying to isolate them.

Nah, say it ain't so! A 300D with leaks? What IS this world coming to?

There is usual a porous metal ring in the base of the reservoir. Think old school type fuel filter. If you siphon/drain the fluid you should see it.

Power steering fluid is NOT required and in fact the shop manual specifically states "Ford type F" fluid.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Mike, probably not entirely a surprise to find a contradiction; an Owner's Manual for 114/115s of 1975 calls for Dexron, rather than Type F.

Nah, no surprise at all.

Depending on the year, I've seen (from the actual Owners' manual) every thing ranging from Type F, Dexron up to the vague, "Any authorized MERCEDES-BENZ dealer will advise you on recommended brands".

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Everybody always argues over fluid. Out of all the threads I've read, the takeaway I've found is that ATF is NOT the same today as what it was when they designed these cars. The gold fluid you have in your res is Power Steering fluid, not ATF. Febi ATF isn't that expensive, it's up to you though.
The filter is here:
1984 Mercedes-Benz 300D Base Sedan - Filters & Belts - Page 3
Note: that link is to an '84 300D, if you look for a '76 and earlier PS filter on pelican and you'll see this:1976 Mercedes-Benz 300D Base Sedan - Filters & Belts - Page 3
Is the filter housing like the one in your example pic? If so, it's like mine and mine uses the later model filter. My engine was replaced in '84 with a rebuilt unit, likely they updated the PS fluid housing.

__________________
63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6