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  #1  
Old 10-09-2017, 01:36 PM
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Master Brake Cylinder?

Dear Forum:

I have just taken possession of my first Mercedes. It's a 1988 300CE. I've owned and rebuilt BMWs & Porsches, so I'm acquainted with German cars from the 1980's. But just let me check in with people who know more than me about W124s.

The brakes worked on the car when it was delivered to me, but the fluid was a dark brown. It looked the color of Maple syrup. I can't remember seeing that before. I suctioned the fluid out of the reservoir to the low mark and added new fluid and then did it again. Then I bled the system, according to the Chilton Service Manual.

When I bleed brakes alone, I use a 2X2 stick. I break open the bleeder and push down the peddle with the stick & hold the peddle down by jamming the other end of the stick against the front of the driver's seat. I close the bleeder and remove the stick....again and again.

It works, but takes a while and in this case, kept me from realizing that I had lost peddle resistance somewhere along the way. Yesterday I was getting fluid out of the bleeders, but the peddle had little resistance and very poor braking by the time I was done.

Today, I enlisted my trusty & faithful wife to press the peddle. By today, the rear calipers were only expelling air with every push of the peddle.

It appears that I have very weak braking from the front calipers...just enough to slowly move the car in my garage. I have no leaks visible from the back of the firewall; no fluid loss into the passenger compartment. The reservoir has always been filled with new fluid during the bleeding process.

So, I think the master brake cylinder has failed. Can anyone think of another possible explanation? Any special Mercedes considerations that I need to know as I prepare to swap in a new cylinder?

Your wisdom is appreciated.

Sincerely, Steve L

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  #2  
Old 10-09-2017, 02:47 PM
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Take close look at the reservoir. Some first timers miss how much to fill the reservoir so both chambers fill. If the initial fill doesn't go high enough, it won't spill over to the empty forward chamber.

Sixto
98 E320s sedan and wagon
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  #3  
Old 10-09-2017, 03:27 PM
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if you went to the floor when you bled the brakes, the master failed. it went someplace it was never before. good luck, chuck.
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  #4  
Old 10-09-2017, 03:48 PM
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Steve, there is a rear chamber in the master cylinder that must be kept full. In the future best to use a pressure bleeder because that will ensure the rear chamber remains filled with brake fluid, and you will avoid getting air trapped in the brake system.
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  #5  
Old 10-09-2017, 03:55 PM
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Your stick method is an interesting idea, but it doesn´t allow for a person to tell you how it feels during the process of opening and closing the bleeder, ie: mushy, solid, etc.

As far as missing fluid, I would check the lower front part of the booster where the master cylinder connects. It is possible to have fluid leak there without notice of it from above. During bleeding when the pedal is pushed to the very floor, fluid can escape the master cylinder and exit at that place. To avoid this, place a short 2X4 on the floor where the pedal would normally touch if pushed completely to the floor.

Of course you should check the MC fluid level after the bleeding of each wheel. It has to always be above minimums front and back chambers.
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  #6  
Old 10-09-2017, 08:36 PM
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Thank you all for your comments. If when I suctioned fluid out of the reservoir, new fluid failed to go into the rear chamber, that would easily explain why no fluid is reaching the rear calipers. Dumb question...on some cars the rear chamber goes to the front calipers. Does the rear chamber of the reservoir go to the rear calipers on the 300e?

But the configuration of the reservoir has the fill port toward the front. When I pull out the screen, I can see that the reservoir is separated somehow into front and rear chambers. But if you pour the fluid into the front chamber, it must fill the rear chamber too.

I just went out to my shop and looked at the reservoir with a flash light. The reservoir looks full from front to rear. I'm not losing any fluid. And when my wife pushes the brake peddle, the master cylinder is pushing air out (somehow). The bleeder tube on the caliper bleeder blows bubbles into the fluid in the cup that accepts the old fluid. I'm perplexed that the pump can blow air instead of fluid, if fluid is in the reservoir.

This car had brakes until I bled the system. I've bled the brake systems on many cars. I've done it 100 times. I think that I'm doing something wrong. I doubt that the master cylinder simply failed while being bled. This is most likely to happen the first time you work on a new car.

It's very possible when I siphoned out the old brown fluid, that I might have somehow allowed the rear chamber to go dry. I was watching the level in the front of the reservoir. But even so, once the reservoir was full, the pump in the master cylinder should take up the new fluid.

I think that I have fluid in the rear chamber. How could you fill this reservoir and not have fluid enter both chambers?
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  #7  
Old 10-09-2017, 11:12 PM
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The master cylinder won't self bleed. If air got into the master cylinder, get a MC bleed kit for lines that route back to the reservoir then 'bench bleed' the master cylinder, best done level. When free of bubbles, go back to bleeding the wheels. Since there are other German cars in the mix, a pressure bleeder is a worthwhile investment. If you're close to SJC you can borrow mine.

Sixto
98 E320s sedan and wagon
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  #8  
Old 10-10-2017, 11:08 AM
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"I think that I'm doing something wrong. I doubt that the master cylinder simply failed while being bled."

like i said, if the pedal went to the floor while bleeding, the master is shot. good luck, chuck.
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  #9  
Old 10-11-2017, 07:20 AM
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Quote:
Originally Posted by porkface View Post
"I think that I'm doing something wrong. I doubt that the master cylinder simply failed while being bled."

like i said, if the pedal went to the floor while bleeding, the master is shot. good luck, chuck.
Maybe I'm misunderstanding you, and certainly the MC may be shot, but the pedal will go to the end of its travel when you press it with the bleed screw open. This is normal. Do you think he may have damaged a piston seal?
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  #10  
Old 10-11-2017, 07:46 AM
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i tell newbies when i use 1 to bleed brakes-put your left foot under the brake pedal, your right on the pedal. when your left foot hurts, you've gone too far. yes, it will bottom out and yes it seems to always ruin the cylinder when that happens. seen it too many times in the past 30 years of doing this as a pro. good luck, chuck.
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  #11  
Old 10-11-2017, 11:03 AM
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I believe the m/c reservoir on 124s is configured like the 201s, and it is very easy to run the rear reservoir dry. You can only see the rear reservoir level at the extreme rear of the res. by viewing from the "inside" (engine not fender side), and it's much tougher to read with clean fluid.

Internal baffles in the res. make it difficult to fill the rear section, and its capacity is small, so it's easy to run it dry. With a helper pushing on the pedal I usually do about 6 stroke per side going around four or five times, and check the rear res level every 12 stokes when working on the rears. A full front res. is good for about double the number of pedal strokes.

Fill the res. to near the top of the opening, then wiggle it back and forth to fill the rear res., and check it frequently as described above to keep track of the fluid level, Don't let it get lower than at least a quarter-inch above the bottom.

When doing a brake fluid change I siphon the front res, but can't reach the rear due to the internal baffling, so I bleed the rear brakes until the rear res is about a quarter-inch above the bottom, then fill the res to nearly the top of the opening and proceed as described above.

I made the same mistake the first time I did a fluid flush over 25 years ago on my 190E 2.6 five speed, figured out the problem, and have proceeded as described above on all subsequent flushes without ever running the rear res. dry again.

Duke

Last edited by Duke2.6; 10-11-2017 at 11:21 AM.
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  #12  
Old 10-11-2017, 02:41 PM
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OK, so I've received a lot of good advice here. I'm new to Mercedes, but have bled a lot of brakes. I can't believe that I allowed the rear chamber to go dry, but that may have happened. Chuck says that if the brake pedal has gone to the floor, it ruins the master cylinder. I always try to keep that from happening, but it may have. Sixto says that the cylinder wouldn't self bleed. I have sometimes simply put a tube on a bleeder and opened it to allow gravity to fill a brake line. That hasn't worked on the 300ce after four or five hours. Normally if you fill the reservoir and break open a caliper bleeder, the fluid will flow.

So, I guess I will get a new cylinder, bench bleed it and put a new one in.

Thanks for all of your help. Does anyone know of a pressure bleeder that fits the W124 that doesn't cost $450?
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  #13  
Old 10-11-2017, 11:01 PM
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The Motive bleeder is about $50. You can make one from a garden sprayer for less. Then there's adapting an air hose to a reservoir cap...

Sixto
98 E320s sedan and wagon
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  #14  
Old 10-12-2017, 09:24 PM
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Definitely get the Motive bleeder (or another brand or a homemade version). I have the Motive and the 1st vehicle I used it on was a W124. The brakes felt brand new when I was done. I never got that same firm pedal pressure when a 2nd person pumped the pedal or when using the stick method (I've done that too). It is really easy to miss the rear reservoir and I nearly ran mine dry a couple of time before I got the Motive.

Factory repair manuals are readily available for your w124:
https://www.startekinfo.com/StarTek/
Index of /service

Good luck!
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01 E320 Wagon (wrecked PM if you need parts), W210, W124, W126
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  #15  
Old 10-12-2017, 10:25 PM
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just keep the master topped to the brim until you're done. Pump out some fluid to get it down to level before you finish the last wheel. I doubt you've damaged the master.

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