R129 RST Hydraulic Fluid Recommendations - Benzworld.org - Mercedes-Benz Discussion Forum

coolkid · 07-26-2011, 11:53 PM

Hi can anyone tell me, what is the specific fluid for the soft top hydraulic for the 129 body?thanks in advance.

Chicky · 07-27-2011, 04:48 AM

Hi Henry,

I replied to your PM.

Are you asking for the MB part number, if yes I will check out my stash of fluids and parts in the storage room tomorrow. I know I have stock, but how many, escapes me at the moment. Do you just need to top off or drain and refill?

Chicky

**Lighthouse** · 07-27-2011, 08:02 AM

coolkid,

As many on this forum know, you should only use the fluids approved by Mercedes, in order to avoid unwanted additives in other fluids attacking the high pressure seals in your system, or dispersing water and letting it hydrolyze the original seals.

The MB part number for the fluid, also called ZH-M, is 0009899103.

In the US, the other approved, widely available fluid is FeBi 02615, made by Bilstein. Here is a link to the picture as shown at Auto Parts at AutohausAZ - OEM Auto Parts - Discount Replacement Parts, Resources and Car Care Tips Part Number Image

Other car manufacturers also recommend a certain Pentosin fluid. Porsche is one of them, and their system uses basically the same materials for seals, pistons, valves, and cylinder bodies. I am hesitant to recommend it here, because it seems that Mercedes has done the most research in the area. They have actually taken another fluid off the recommended list way in the past, because it had a dye in it that was not compatible with the seals in the hydraulic system. The following thread illustrates in response #14 what happens if the wrong additives are used in your fluid: http://www.benzworld.org/forums/r129...hydraulic.html

There is a fluid made by Aral in Europe, and it would be great to get input from our members over there if anyone has used or bought it.

I know from customers "down under" that the MB fluid there is made by yet another manufacturer, I think it's Weyle - folks, please correct me if I'm wrong. However, their fluid still carries the MB part number.

-Klaus

klaus@tophydraulicsinc.com

**Lighthouse** · 07-27-2011, 08:18 AM

Since coolkid just had all the cylinders upgraded, now is the perfect time to get all the old fluid out of the system. Old fluid should be flushed in general, because it carries abrasive particles, and the old fluid becomes in general more aggressive towards the seals in the system. There are threads on flushing in the stickies, and I don't want to override them here.

My point is, though, that you will get about 95% of the old fluid out of the system at this point (after the old cylinders had just been removed) by dumping all the old fluid out of the reservoir before moving any of the newly installed cylinders. The system holds over 1 quart, so you will need to buy 2 quarts of fluid.

That should be good enough for systems that have not had any harmful "Stop-Leak" type additives, or water, or other unapproved fluids in them.

I'll post a follow-up shortly, showing what the wrong fluid can do.

-Klaus

klaus@tophydraulicsinc.com

**Lighthouse** · 07-27-2011, 09:12 AM

The OEM rod seals in the hydraulic cylinders are the only ones in the system that will normally fail. By normally failing, I mean chemical decay that cannot be prevented even with the perfect hydraulic fluid. These are certainly the most sensitive seals in your hydraulic system, but there are plenty of others that can suffer from the wrong hydraulic fluid, including piston seals, line seals, valve seals, and the pump. You do not normally have to worry about any of the latter, if you use only the proper hydraulic fluid and change it every few years.

The rod seals are cup seals that are made of Polyurethane, which is susceptible to water and to certain solvents. One of the traits of MB approved liquids is, that they do NOT immerse/disperse water in the fluid, so that any water from condensation eventually collects on the bottom of your hydraulic fluid reservoir, where it will not attack the seals.

Many transmission fluids and basically all gear oils have additives in them that soften seals and make them swell up to some degree. This is supposed to keep the transmission housings and the gear cases from leaking. The problem with regular/OEM Polyurethane cup seals is, that they will get to catastrophic failure pretty soon after exposure to such additives. Typically, they will either turn into some gel after all that "softening", or they will dry up and crumble even faster than in the regular chemical decay.

One of the dangers with crumbling seals is, besides the basic leaks and resulting mess and loss of operation, that the seal particles can clog up all the filters in the system, including the individual filters in the many solenoids in the hydraulic valve blocks. That can result in slower operating speed of your top even after you have replaced or upgraded the cylinders.

Since there have been plenty of pictures with cracked and decayed seals in this forum already, I'll show some with exposure to water below. The cylinder had been rebuilt by our friendly competition just over a year ago. Unfortunately, that rebuilder uses basically the same material as MB. To be exact, it is practically the same material that MB has been using in their early SLK models. The photo shows how the seal has swollen up from hydrolization: it has become about 20% longer, and the sealing lip on the OD is now protruding out of the aluminum cap it was supposed to seal. The seal has become very soft, and the next stage would be disintegration. Just for reference, I'll show some other seals of the same make in different stages of decay in the last picture.

The poor customer also had to replace the pump.

-Klaus

klaus@tophydraulicsinc.com

grumpy old man · 01-29-2013, 08:34 AM

I need to top off my top resivoir. I have on my shelf some oil for hydraulic suspention. I do not have hydraulic suspension so it was either purchased for the power steering or the top. I would think that the suspention and top would share the same fluid but before I dump it in I would like to know for sure and not screw something up.

**bayhas** · 01-29-2013, 08:42 AM

Quote:

Originally Posted by grumpy old man

I need to top off my top resivoir. I have on my shelf some oil for hydraulic suspention. I do not have hydraulic suspension so it was either purchased for the power steering or the top. I would think that the suspention and top would share the same fluid but before I dump it in I would like to know for sure and not screw something up.

Yes, same fluid, MB part number is 000 989 91 03

**lynns** · 01-29-2013, 08:59 AM

Quote:

Originally Posted by grumpy old man

I need to top off my top reservoir. I have on my shelf some oil for hydraulic suspension. I do not have hydraulic suspension so it was either purchased for the power steering or the top. I would think that the suspension and top would share the same fluid but before I dump it in I would like to know for sure and not screw something up.

You mentioned three different systems. The soft top, the suspension, and the power steering. Please don't confuse the fluids that go in them. One size does not fit all. The power steering fluid is a different p/n from that of the soft top hydraulic fluid. Please see the attached PDF from MB.

sapphirexae · 01-29-2013, 11:22 AM

First off, there is a reason your fluid is low. The fluid does not evaporate. If you haven't already looked/replaced/repaired any of your cylinders......you have a leak somewhere.

**bayhas** · 01-29-2013, 11:50 AM

Quote:

Originally Posted by sapphirexae

First off, there is a reason your fluid is low. The fluid does not evaporate. If you haven't already looked/replaced/repaired any of your cylinders......you have a leak somewhere.

Very good point, and I will add, that fluid is fairly abrasive, and interior leather and plastic dont mix well with it AT ALL, in case the leak is in the upper windshield pistons...