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  #16  
Old 07-01-2011, 09:24 AM
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LEATHERMANG, I'm not too old to learn but I should have inserted the word "usually" into my sentence. I think the pushrod went too deeply into the master and deformed/ tore the seals.

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  #17  
Old 07-01-2011, 10:11 AM
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Are you getting good flow out of each wheel? I usually bleed mine at about 25 psi. Your presure bleeder has an external container for barke fluid so that neither chamber goes dry right?
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  #18  
Old 07-01-2011, 10:17 AM
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If you only bled the front calipers I bet you got air bubbles in the lines to the rears and it does not take much air to get the pedal to sink. Removing a caliper allows the entire line to empty right to the "T" where the rear and front for each circuit connect. You can easily get air in the rear line at the "T" which can cause problems. The brakes generally aren't split into front and rear circuits but rather diagonally (right front/left rear and left front/right rear) so if one circuit fails you still have one brake on each side - I am pretty sure your car is set up that way too which means you actually opened both circuits and should have bled all of the lines.

Try re-bleeding all 4 wheels, starting at the farthest and working to the nearest, then report back. If that doesn't do it then you probably did trash the MC by pumping too far...it is a common mistake which even some shops and dealerships do.
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Last edited by nhdoc; 07-01-2011 at 10:27 AM.
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  #19  
Old 07-01-2011, 11:50 AM
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On my 123 anyway, the brakes are split front and rear. The rearmost chamber in the MC reservoir is for the fronts, and the front most part of the reservoir is for the rear brakes. There is a wall that separates the chambers in the reservoir, and you have to fill the front chamber up to the top to get the fluid to flow over the top of the separator into the rear chamber. At least on this car, you could get away with changing out either fronts or rears and not needing to bleed the others if you were careful with the fluid.
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  #20  
Old 07-01-2011, 12:51 PM
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Quote:
Originally Posted by sokoloff View Post
Are you getting good flow out of each wheel? I usually bleed mine at about 25 psi. Your presure bleeder has an external container for barke fluid so that neither chamber goes dry right?

Correct, the fluid container doesn't go dry at any time with the power bleeder.
I'll try and bleed all 4 wheels with higher pressure.
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  #21  
Old 07-01-2011, 05:21 PM
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Quote:
Originally Posted by woodrat View Post
On my 123 anyway, the brakes are split front and rear. The rearmost chamber in the MC reservoir is for the fronts, and the front most part of the reservoir is for the rear brakes. There is a wall that separates the chambers in the reservoir, and you have to fill the front chamber up to the top to get the fluid to flow over the top of the separator into the rear chamber. At least on this car, you could get away with changing out either fronts or rears and not needing to bleed the others if you were careful with the fluid.
I guess I have seen it both ways, diagonal and front/rear...not sure which system Mercedes used but if your W123 is that way then his probably is too in which case you would not have to bleed the rears if you didn't open that circuit.
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  #22  
Old 07-02-2011, 01:49 PM
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update


1) I did another round of bleeding ....1 liter started rear , the front...1 liter.
No air bubbles ....no change. pedal goes down.

2) I did vacuum test on brake diaphragm. It is holding vacuum (I tested 15 inch)
for over 4 hours now. So the diaphragm is fine I believe.

Result...it must be the Master Cylinder!

So can someone explain me what happens if the Mastercylinder fails like in my case ...pedal sinking?

How difficult is it to change the MC?
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  #23  
Old 07-02-2011, 02:18 PM
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Fluid goes by the seals--into the booster usually. Not good. Pretty easy to swap out a MC. Bench bleed it before installation and no bleeding should be necessary.
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  #24  
Old 07-02-2011, 02:25 PM
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I do not believe anyone is good enough to connect the bench bled mc to the lines without having air in the junction... not much.. might could be ignored...but probably should be bled ..
and if your mc has put brake fluid into the brake booster...need to correct that...
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  #25  
Old 07-02-2011, 04:34 PM
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What I read is the column of fluid at the far end of the master cylinder doesn't circulate well since you don't push the pedal to the floor on any regular basis. Moisture in that blob of fluid corrodes the master cylinder bore. it only becomes a problem when the pedal goes to the floor such as when bleeding. The piston seal slides over and is damaged by the corroded section of bore. I don't know that this should necessarily cause fluid to leak into the booster. Seems it will act more like a worn shock absorber with no external leaks. Of course a master cylinder can leak from the actuator end into the brake booster. Would that go into the vacuum side of the booster?

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  #26  
Old 07-02-2011, 10:27 PM
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Quote:
Originally Posted by leathermang View Post
I do not believe anyone is good enough to connect the bench bled mc to the lines without having air in the junction... not much.. might could be ignored...but probably should be bled ..
and if your mc has put brake fluid into the brake booster...need to correct that...
Leathermang:

'need to correct'- how to correct?
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  #27  
Old 07-02-2011, 10:36 PM
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You would not want to leave brake fluid in there in contact with the rubber diaphram ... thus either taking it apart and cleaning it well.. or replacing it would be standard operating procedure...
And if you clean it .... or put a kit new one into yours.. typically you coat it with talcum powder .. but check the instructions...
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  #28  
Old 07-02-2011, 11:07 PM
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Quote:
Originally Posted by sixto View Post
What I read is the column of fluid at the far end of the master cylinder doesn't circulate well since you don't push the pedal to the floor on any regular basis. Moisture in that blob of fluid corrodes the master cylinder bore. it only becomes a problem when the pedal goes to the floor such as when bleeding. The piston seal slides over and is damaged by the corroded section of bore. I don't know that this should necessarily cause fluid to leak into the booster. Seems it will act more like a worn shock absorber with no external leaks. Of course a master cylinder can leak from the actuator end into the brake booster. Would that go into the vacuum side of the booster?

Sixto
87 300D
My master cylinder was old and crudded up like this, and it had no external leaking going on, just cruddy, wet, old fluid sloshing around inside, leaking past the seals and not doing any useful work.

And yes, if it leaked out through the back of the MC into the booster, it would be leaking into the side of the booster with vacuum on it.
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  #29  
Old 07-04-2011, 09:11 AM
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Quote:
Originally Posted by sixto View Post
What I read is the column of fluid at the far end of the master cylinder doesn't circulate well since you don't push the pedal to the floor on any regular basis. Moisture in that blob of fluid corrodes the master cylinder bore. it only becomes a problem when the pedal goes to the floor such as when bleeding. The piston seal slides over and is damaged by the corroded section of bore. I don't know that this should necessarily cause fluid to leak into the booster. Seems it will act more like a worn shock absorber with no external leaks. Of course a master cylinder can leak from the actuator end into the brake booster. Would that go into the vacuum side of the booster?

Sixto
87 300D
Thanks Sixto,
What you are saying make the most sense to me. I believe this is happening in my case.
Why should the seal to the diaphragm leak all of a sudden?
I think I won't see any fluid in the booster.
Let's see when I am getting there. I am waiting for new Ate Master Cylinder.
It was maybe about getting time anyhow after 330 K miles.
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  #30  
Old 07-04-2011, 10:35 AM
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If so, better that it happens at home than during a panic stop.

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