I want to flush the ABS unit. The pressure method, with about 29 pounds of pressure, supposedly will open the valves inside the ABS & allow fluid to pass through.

I don't trust 'supposedly' and have minimal trust in shops that say they will flush the ABS. No way to verify that has been done. That is why I want to do the work myself. I'd like to avoid the ABS unit going bad due to fluid degradation & possible corrosion.

It is an expensive unit and appears to take some time to install a new one properly. I'd like to try to avoid that from happening, if flushing the ABS will reduce the chance of failure.

Does anyone know if the pressure method will flush the ABS? Is it documented anywhere? Without being able to directly drain the old fluid from the ABS until it flows good and clean, how would one ever know that fluid has been changed?

Go through all the posts on this board an all the posts on all the Mercedes boards on the net, and find one, just one, unambiguous example of hydraulic ABS failure.

If it's pressure you want, physics is your friend. Do you have any idea how much pressure can be developed by the foot pedal? It's in excess of 1000 psi. It's the product of your leg strength (say 100lbs to start), the lever ratio of the brake lever, and the ratio of the master cylinder diameter to the line ID. And that's not considering vacuum boost. It's more than enough pressure to push old fluid and contaminants out of the entire brake system.

Thanks to MHUNTER's link to a startek site, I was able to find the Mercedes method which does use a pressurized ( about 30 lbs ) container of brake fluid attached to the top of the master cylinder.

https://www.startekinfo.com/StarTek/outside/12264/disc_2/Program/Chassis/42-0010.pdf

His post gives a lot of information about service manual sources including Mercedes provided links. I used their one for the W124.

Mercedes Service Manual Source, CD/DVD/Paper

Now I've got to read about the 'master cylinder switchover solenoid valve' & why that has to be energized. Some say it doesn't have to be energized others say it does.

I also haven't found a definite statement that the ABS can be flushed without energizing the ABS solenoids.

I can't think of a reason an ABS unit won't fail especially if old contaminated fluid is kept inside the unit.

After several hours of searching I still couldn't find any definite statement that pressure flushing will also flush the brake fluid from the ABS unit if the ABS solenoids are not engaged. I've sent an email to one of the major rebuilders asking them if they know. Regrettably Bosch won't answer the simplest of questions ( like what lubricants to use for specific Bosch products ).

I have found some information by searching:

- glycol type brake fluid absorbs about 2 percent of water by weight each year, supposedly the rubber brake lines allow most of that water vapor to enter.
- after a couple of years the boiling point of the brake fluid drops by about 100 degrees. This can cause loss of braking due to the water turning to vapor resulting in up to complete loss of braking at the time you're attempting an emergency stop.
- some ABS failures result in complete loss of braking. Search ' Mercedes ABS brake failure '
- Mercedes sensotronic had a much higher than expected failure rate.
- mycardictionary.com has plain english explanations about brake fluid and other common topics.
- some later model mercedes braking systems have a brake activation counter built in. Even if the brakes are still working good it appears the expensive brake system component has to be replaced.
- one article said to expect only about 7 years of life for newer cars ( because they get too expensive to maintain after that ).
- some ABS systems actually provide more pressure than what the master cylinder produces
- ABS seldom engages to help safely brake the vehicle, supposedly less than 2 percent.
Most stops only use the master cylinder pressure without interference by the ABS. Critical to have it working if driving on slippery surface due to rain, oil, ice, sand, etc...

One really good downloadable article can be found by searching for 'ANTI-LOCK BRAKE SYSTEM Article Text'. It is copyrighted by Mitchell International but seems to be ok to download for personal use. This describes the Bosch ABS & troubleshooting with diagrams for a 1989 Benz 190E. Probably close to my 1995 E300D.

Is there any particular reason this job is being so over-thought and over-complicated? Flush the damn lines and master cylinder and go have a beverage. Having ABS pump failures on these old Mercedes is so rare you almost never hear of it. Most problems with ABS is the electronics, not the pumps. Failure of the pump comes when people NEVER change the fluid, and I mean for decades or longer. Of course they also trash the MC and the calipers, but hey, it's their money.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Been that way most of my life. Far to many opinions that are fluffy and only minimally correct. I just like to know things & will spend hours and weeks trying to find enough information to satisfy me. Quite often I have to give up because I haven't talked to the right person or found the information through the internet.

One example that baffles me is medical treatment related. Most people assume their doctors will do the best for them. In chemotherapy using 5FU for certain cancers there is a test that can be given before the drug is given. This test can determine if the drug itself will potentially kill the patient in a matter of a couple of days after the first dose. Doctors typically won't test if the patient has that sensitivity. The chemo kills/seriously injures the patient. Also, the first dose is quite often the 'maximum' dose to be given not a small dose to see if there is an unexpected reaction. Cancer was the lesser problem because the test was not given because it is not in ' the standard of care '. My 85 year old neighbor is alive today partly because of my reading about his medical treatment. Transferring to a more professional treatment center provided him with immediate health improvement. That center over a two year period has provided him with a ' cancer free ' diagnosis for the 1st time in about 7 years.

Cancer treatment and vehicle maintenance are worlds apart. Let's not confuse the two.

When it comes to maintenance of any mechanical item, usage and life expectancy should be factored into the equation. Things like brakes have a finite life and a recommended service interval that's been established for many decades. Change the fluid, replace the pads when worn, machine the braking surface when out of tolerance. When something wears out from use, it is replaced. Most other types of failure are from neglect.

Obsessing over every little detail will lead to a heart attack or a stroke with very little return on investment (both in time and money). Tearing things down and worrying about things that aren't issues can often lead to an issue that wasn't present to begin with. There's an entire science devoted to it, if you want to read more just google "Infant mortality equipment failure" (it has little to do with infants) or "Bathtub curve".

Flush the fluid with a decent quality fluid and move on with life. In 2-3 years, flush the fluid again and drive on worry-free. There are plenty of us with 30-40 year old ABS pumps original to the car that still work perfectly all these years later. It's literally the least of your concern in the braking system on these cars.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

You tube videos for all ways to do this job . Makes things easy just looking at the job done on youtube. Me i will just purchase 2 litres of MB fluid remove all the old fluid in the catch tank with a syringe ,then refill with new . Do it as many times as you have fluid to spare ... That way no mistakes , and you will always have brakes.

To me, both a medical condition and a mechanical condition can each harm or kill. Sometimes it's others and sometimes it's you. If you only have an opinion the lack of accurate knowledge can cause unintentional harm.

I've seen most of my friends and relatives follow their doctor's advice without taking any time to research what is happening to them. Most recover just fine. Some don't. I just returned from my elderly (85 year old ) neighbor that has a massive sinus infection. His doctor is ' too busy ' to schedule the cleanout until the 28th of August. So he's on antibiotics and steroids til then. He has severe headaches and really nasty looking discharge. To me that is stupid for both the doctor and my neighbor to wait that long. I'm old enough to have seen friends and relatives die due to lack of knowledge or concern.

The same is true of brakes on our vehicles. Most have an opinion, very few have taken time to see what hidden problems they might have.


Too many think 'Never had a problem, so I'm not concerned about it'. I don't change the fluid, seals in master cylinder, calipers, and definitely don't send in an ABS for a checkup and reconditioning. The age of the vehicle doesn't seem to be part of the consideration, the ABS and the brakes are just expected to work.

For the majority of drivers they don't realize that old water logged fluid can cause their brakes to fail when they need them most. Their vehicle can experience brake failure due to the water vaporizing or a corrosion pocket/sediment causing loss of braking pressure due to seal failure, orifice restriction, line restriction or bursting of the line.

2. Some actually compress their rubber hoses with vise grips to keep the fluid from coming out. They don't think that might weaken the hose or lead to a blockage when the liner inside fails.

3. In my recent reading/research it appears that the ABS unit is not serviced until it fails. As indicated by the comments here, it is one of the components that people expect to work because it seldom fails. The ABS control does run checks on the ABS before the vehicle gets up to speed. Supposedly the ABS is not activated if the ABS fails one of those checks. Undetectable failures can cause reduced braking, loss of braking, or pulling in one direction or the other due to an imbalance of pressure coming out of the ABS.

From my reading I've discovered that:

1. The ABS has paths that are not flushed by normal or pressure bleeding.
2. There is one or more filters inside the ABS that can restrict or block fluid flow if they catch any debris. These filters are not cleaned during pressure flushing because they are in the path of fluid flow. They are only cleaned or replaced during a rebuild/cleaning procedure. The only way I can think of to clean them would be a reverse flush under pressure to dislodge the particles caught by the filters. Never heard of that being done for that purpose.

The solenoid valve(s) for each caliper can restrict, block, or allow full flow of the master cylinder pressure. To me corrosion, sediment, debris could mess with the solenoid valve movement.

3. The ABS control is voltage sensitive, the ABS can stop working if the alternator is not capable of handling a momentary high demand for output.

4. I had never heard of doing a brake fluid pressure check at the calipers, master cylinder, and other locations.

The following are a few of the links that provided some information on braking:

https://www.brakebleeder.com/pressure-testing-brake-hydraulic-system-tips/

http://www.mye28.com/viewtopic.php?t=109879 informative pictures start March 22nd 2015. Shows contamination and talks about filters after that.

https://pe2bz.philpem.me.uk/Misc/-%20Auto/-%20Pdf/absmb190e.pdf one of the sites that has a PDF file that covers Mercedes ABS for a few models in the 80's. Seems to be the one that matches my 95 E300D.

https://www.brakebleeder.com/preventing-damage-abs-modulators-brake-pad-replacement/ one discussion of ABS internal filters

Lengthy but I found this while searching for ABS motors information. Shows current draw and waveforms for most auto electric driven devices. Primarily for vehicle 'sudden acceleration'.

https://www.autosafety.org/sites/default/files/imce_staff_uploads/Belt%20An%20Electronic%20Cause%20for%20Sudden%20Unintended%20Acceleration.pdf

I think you're going overboard. I just flushed the brakes on my E320 Wagon, easy as pie.

You're making it extremely difficult, how would you even reverse flush it? It would require even greater working pressure correct? It also require custom lines and connectors right? What if you screw up, how will you get the air out?

Also would reverse flushing ruin the master cylinder seal?

__________________
1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

Reverse flushing, whatever that means, shouldn’t bother the seal since you wouldn’t flush with more pressure than the system sees during a panic stop.

Aside from regular fluid changes, check your ABS in a controlled environment. That’ll either convince you it’s working or tease out an imminent failure.

Sixto
98 E320s sedan and wagon
02 C320 wagon

95 E300D

With some searching today I've found a bit more that confirms my thinking that proper checking, flushing, and preventative maintenance of the ABS is severely lacking and does present a safety issue.

I've found a Porsche thread that discusses the Bosch hydraulic pump that I believe is the same one my car has.

The link is:

https://rennlist.com/forums/964-forum/853080-964-abs-pump-cheap-repair.html

The thread mentions a Bosch supplied ABS tester that might be capable of testing some of the ABS units like the one on my car. However they mention that it might just test the relays.

The posting members discuss failures of their ABS & a few were able to ' fix ' their issues. Others were not.

This is as I suspected a ' hidden ' problem because most of us just expect the ABS to work and the brakes to safely stop our cars. I haven't found one mention of the innards of an ABS to ever be routinely inspected by a shop for possible problems that might cause future brake failure.

I'm concerned because I want to be able to trust my car while driving on long trips & in different types of weather.

I've not been able to find a company that has a service to clean and rebuild the hydraulic portion of the ABS. They all appear to focus on the electronic control portion.
The above thread primarily focuses on the corrosion and seizing of the pump motor pistons. They suspect it is due to the lack of routine cycling of the ABS during most of their driving.

They've built a crude but effective way to exercise the ABS motor and the solenoids. They also mention a book that probably has more detailed information about the ABS. Later this week I'll search and try to get online access to the content of that book.

It appears to me that this is probably an unrecognized world wide problem due to people assuming that their brakes will function as designed. Probably the cause of accidents/wrecks is not researched deep enough to highlight the failure of ABS controlled brakes on older vehicles. It is clear to me now why Mercedes has provided a cycle count on some of their ABS units, they know they are susceptible to random failure after they age. Probably might even be best on older ABS equipped vehicles to consider completely bypassing the ABS unit to avoid brake failure due to the corrosion, contamination, filters blocking flow, and solenoid failure inside the ABS unit. If it is a fair weather car the ABS wouldn't be kicking in anyway.

What I intend to do, when I have time, is to acquire a few ABS units that match mine and learn how to disassemble, clean, test, and possibly repair them. Then I'll tackle the one on my car. At least I'll have lots of donor parts.

I spent very little time to find out the Streather book is a lengthy one that is titled:
Porsche 911 (964) Enthusiast's Companion

It must go into great detail about that Porsche model if he includes the current required to activate the ABS solenoids.

I also searched for the Bosch 2-led tester and found a few references to it. One is at:

http://www.w124performance.com/images/tools/Bosch_ABS_tester_KDAS-0003/Bosch_ABS_tester.pdf

Description of how to use the Bosch 2-led tester:

https://www.porscherepair.us/hydraulic-unit/mar.html

unbelievably long procedure, guess that indicates how complicated the system is.

The airbags and pyrotechnic pretensioners are beyond their shelf life. You’re making a case for a later model car or a pre-84 without ABS or airbags.

Sixto
98 E320s sedan and wagon
02 C320 wagon

The best way to test ABS is to brake on gravel or wet asphalt (big empty parking lot, please) . You want to induce the wheels slipping.

I just flushed my fronts on Saturday with my Motive pressure bleeder then drive to the farm. Gravel driveway worked wonders and the car actually stopped well on it.