This is what my old fluid looked like. It had the appearence of graphite not clear or even cloudly like beer. After cleaning the container I put one bottle of "No Leak" and filled the container and flushed out the system till the fluid cleared out. Then I topped it off with more fluid. 10/01/2008
Why you should flush the convertible top hydraulic fluid
Unless I missed earlier discussions, I see no mention of the danger of allowing moisture to get into the system. The hydraulic fluid is hygroscopic meaning that like brake fluid it absorbs moisture from the air. The polyurethane seals although resistant to moisture are not able to last forever and do fail due to exposure to water. Seal experts tell me that a combination of water and heat lead to a rapid kind of seal deterioration that resembles cottage cheese.
A picture of a top lock seal that shows the "cheese" like texture at the failure points is attached.
Others have pointed out in this forum that the original seal material is based on a subgroup of polyesters called polycaprilactones that have improved resistance to moisture and may be the reason for its use.
What all this may mean is that if you don't change your fluid as often as you do your brake fluid (two years?) you will have moisture in your system and probably are cheesing your seals both in the cylinders and in the pump.
You have not found any ph related threats so maybe we should shift attention to the water.
By the way, note that some later model Mercedes reservoirs have a big filter cap that appears to contain silica-gel. Ah ha Mr. Holmes, we may have a clue here.
For those that don't know, silica gel is used to absorb moisture.
Thoughts on how flushing works on this hydraulic system. (Based on the service diagrams)
I think the system flushes each cylinder to the reservoir on both strokes of the piston. A short way of saying it is on each stroke, the unpressurized side of the piston is connected directly to the reservoir. By working the cylinders several times, you should change out the fluid if you are catching the return flow in a jug and not letting it back into the reservoir.
Fact, Mercedes hydraulic fluid Part#000989910310 is not at all like brake fluid which is hygroscopic. The Mercedes hydraulic fluid is nothing more than Mineral oil which is not hygroscopic or does not attract moisture. Hydraulic fluids are a large group of fluids. I believe when one mentions hydraulic fluid we automatically
think of brake fluid. If it were like brake fluid it would destroy our seats and painted objects ect.
Brake fluid is a subtype of hydraulic fluid with high boiling point and low freezing point. It is hygroscopic, so that it will absorb water which could otherwise cause corrosion of brake system components.
From Wikpedia
Mechanical and industrial
Mineral oil is used in a variety of industrial/mechanical capacities as a non-conductive lubricant. Refined mineral oil is used as transformer oil. Electric space heaters sometimes use it as a heat transfer oil, and it can be used generically as a coolant in electric components as it does not conduct electricity.
Because it does not absorb water from the air, mineral oil can be used as an automotive, aviation, and bicycle brake fluid.
Used as a Preservative
Since it does not absorb atmospheric moisture, mineral oil is useful as a protective coating or bath for water-sensitive materials. Mineral oil is also often used as a coating on metal tools and weapons, knives in particular, as a way to inhibit oxidation.
Last but not least, I would change the hydraulic fluid at least once a year due to the small particulates in the oil from use. Mercedes Benz of Ft. Lauderdale has the OEM Part#000989910310 hydraulic fluid for $11.31 plus shipping.
Does it have to be Febi-Bilstein hyd fluid? Or would any good brand of hydralic fluid work?
We have discussed this at length and determined you can use whatever you want but if you want to follow what M-B has designed to work with the system you will use M-B p/n 00098910310. Febi-Bilstein 02615 was determined to be the same mineral oil as the M-B oil and it saves you money. So I say buy whatever suits you fancy. I chose Febi-Bilstein 02615 and have had NO problems.
Please keep in mind M-B doesn't suggest a change of this fluid at any point in there maintenance schedule. But if you were to look at several of the old oils that have come out of cars, that have needed replacement seals, you would see a dark color, just like the color of the seals.
The only reason I wrote this DIY was as a proactive measure that requires less than one hour to perform and it's as easy as changing the engine and cabin filters.
I was just on the phone with one of our forum members last night and he updated me on his current testing of the replacement seals that match the compound that M-B uses. After much discussion with his conterparts in the engineering business he too was looking for a different seal compound as well as other ways to approach the problem. This guy is great, he went to a M-B junk yard and went as far as to buy each of the cylinders, various seal componds, etc. and determined, thru reverse engineering (this is his profession) that M-B had it right the first time. So if we have what we have, approximately 8-10 years on the front 2 cylinder seals, probably due to heat and /or water, and and we have not had that many report the replacement of ALL of the remaining 10 cylinders as yet. But mostly the are MB90-95. If you plan on keeping you car it would be wise to think proactively and buy a complete set of 12 cylinders worth of seals with a little mark up for postage and handling. Then you would be good to go on any DIY cylinder, less maybe the roll bar. And that is still in question.
Believe me when I tell you that it has not been easy to find the correct seals. And they need to be bought in a bulk numbers of 500 each in order to really save. Yes, some people use seals that they pull from there bicycle pumps, etc. But I would prefer to use the correct size and compound for another 10 years if I'm going to go through thru all of DIY laborl. BTW, a M-B Dealer would charge you more than the car is worth to replace ALL your cylinders, not rebuilt, for about $12K vs. $150-$200 for a complete set of seals for the DIY'er.
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