Brake Fluid Flush, W124 - PeachParts Mercedes-Benz Forum

Oldwolf · #1 08-24-2009, 11:34 AM

I am planning to flush the brake fluid in my W124.128. Now, on my American cars with ABS, I first suck out the old brake fluid in the reservoir, refill with fresh and then proceed with my MityVac to bleed the brake cylinders until fresh fluid come out, always being careful to keep the master brake cylinder reservoir full.

This method has worked well and I have not bothered with flushing the ABS unit.

On my MB, I am wondering if this same procedure will be satisfactory or if I need to flush the ABS unit too?

I see in the service manual where there is a drain port (marked “SP”) on the ABS pressure unit where I assume I can pull a vacuum and drain the old fluid out, while keeping the main reservoir on the master brake cylinder full. At least I think this is what “SP” is for.

So my question is if it is acceptable to only drain the old brake fluid from the caliper cylinders and ignore the ABS pressure unit?

vstech · #2 08-24-2009, 11:45 AM

in a word? NO.
vacuum bleeding is a pain and it's not recommended.
the best way is first go to your local auto parts store, and pick up a GM master cyl. reservoir cover. next get a 1/4" threaded barb hose adapter. next drill a 7/16" hole in the center of the cover, and thread the adapter in with some JB weld on it, let it harden, then get a cheapo garden sprayer, and some brake fluid of choice. (I use VALVOLINE dot 4 synthetic) anyway, cut the end off the sprayer wand and use a short piece of fuel hose or what have you, to connect the wand to the barb adapter.
now fill the sprayer about 1/2 way with brake fluid, and screw on the cover, and pump it up a few times. not many, just 5 or 10 pumps, and go to the back caliper and open up the bleeder, and when it flows clear, close it, and open the next and repeat until all flow clear. release pressure on the adapter, use your mityvac to suck the level in the reservoir down to the full mark, and you're done.

Oldwolf · #3 08-24-2009, 11:50 AM

But you ARE saying I don't need to address the ABS unit individually. Right?

Pressure bleeding probably is better but I have not found vacuum bleeding to be that big of a pain.

vstech · #4 08-24-2009, 11:52 AM

oh, but pressure bleeding is SOOOOOOO much easier and simple to do.
anyway, no. the abs system will be flushed out with the pressure bleed method.

how do you vacuum bleed without getting air in the system? my bleeders always leak!

Oldwolf · #5 08-24-2009, 11:58 AM

Well, I make sure the booster reservoir is always full. That's the only place where you can get air in to the system in my experience.

babymog · #6 08-24-2009, 11:59 AM

Since the bleeders leak at the ouput, the air goes into the removed fluid. The bleeders are also at the high-point of the calipers.

Vacuum-bleeding is what I use, same method as in your OP. Start with the longest run.

I also use a fillet of Dow high-vacuum grease (a peanut-butter consistancy silicone) on the bleeder threads to help the efficiency of the vacuum-bleeding process.

Bleed every 2years and you'll be fine. Not a bad idea to replace just the reservoir fluid annually just to keep it clear IMO.

compu_85 · #7 08-24-2009, 12:02 PM

On Bosch ABS2 systems (like what your car has) there is no extra work that has to be done to bleed the ABS valves. Cars with ASR or later ABS systems do need a scan tool to get fluid moving through.

Just don't run the master dry and you'll be fine

I'll also 2nd the pressure bleeder... once you use one you'll never do it any other way!

-Jason

#8 08-24-2009, 12:27 PM

I would prefer pressure bleeding or two man method over using a vac at the calipers by a margin of about 100 to 1.

Chris W. · #9 08-24-2009, 12:33 PM

+1 with Larry. I used a Miteyvac for years, until I got a Motive Power pressure bleeder. Much easier, no comparison. Just do it.

Rgds,
Chris W.
'95 E300D, 294K

sixto · #10 08-24-2009, 02:18 PM

To save a few 'Help!' posts, the reservoir has front and rear chambers. The chambers empty at the same rate when you extract fluid but fill at very different rates. Wait for the rear chamber to fill as you fill the front chamber before you begin vacuum bleeding. It takes minutes, not seconds.

Sixto
87 300D

compu_85 · #11 08-24-2009, 02:26 PM

When I flush brakes that haven't been flushed in a long while I like to pull the screen out of the master res, suck most of the fluid out with my Pella pump, then wipe as much of the goo in there off as possible, being sure not to leave any chunkies that could get stuck in the ABS system. I then refill and bleed with cheap DOT 3 fluid (new DOT 3 fluid has a higher boiling point then ~3 year old DOT 4 fluid). Then I drive for a while as all the crap still sitting in the system gets absorbed by the DOT 3. Then, I rebleed with some quality DOT 4.

-J

Oldwolf · #12 08-24-2009, 02:37 PM

Quote:

Originally Posted by sixto

To save a few 'Help!' posts, the reservoir has front and rear chambers. The chambers empty at the same rate when you extract fluid but fill at very different rates. Wait for the rear chamber to fill as you fill the front chamber before you begin vacuum bleeding. It takes minutes, not seconds.

Sixto
87 300D

Thanks for the tip.

#13 08-24-2009, 03:18 PM

Quote:

Originally Posted by compu_85

When I flush brakes that haven't been flushed in a long while I like to pull the screen out of the master res, suck most of the fluid out with my Pella pump, then wipe as much of the goo in there off as possible, being sure not to leave any chunkies that could get stuck in the ABS system. I then refill and bleed with cheap DOT 3 fluid (new DOT 3 fluid has a higher boiling point then ~3 year old DOT 4 fluid). Then I drive for a while as all the crap still sitting in the system gets absorbed by the DOT 3. Then, I rebleed with some quality DOT 4.

-J

What you describe sounds more suited to rejuvenating a neglected system. If you flush annually you will not have to go to such extremes.

compu_85 · #14 08-24-2009, 04:30 PM

Larry, I agree 100%. When you change out the fluid every 2 years it never gets to the point of needing a double flush routine.

-J

gsxr · #15 08-24-2009, 08:04 PM

You need to bleed the ASR pump separately on W124 models with ASR. But you do not need to do anything special on a car withou ASR - just evacuate the MC reservoir, refill, and bleed all four calipers. The "SP" port only exists on models with ASR. As mentioned above, I very strongly recommend pressure bleeding, and NOT using a vacuum method. I don't believe the vacuum method works at all on cars with ABS, due to valving in the pump assembly... pressure is required. You can use the "pump the pedal" method, just don't press the pedal past halfway down on each stroke.

Factory procedure for bleeding the brakes (through 1993) :
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Maintenance/My81/4280.pdf

Factory procedure for bleeding the brakes (from 1994) :
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Maintenance/94_95/4280.pdf

If you don't have a pressure bleeder, you can either make one, or buy the Motive Euro pressure bleeder for $50. It pays for itself with one use. Don't forget to use DOT4 fluid... Valvoline Synthetic is the best bang for the buck, IMO, and commonly available at your local McParts. I prefer Ate SuperBlue or Type 200 if available (but it costs almost twice as much).