Good evening Gentlemen,

The brakes on my '85 300D Turbo W123 works but fading away when I step on the pedal for...lets say, more than 5 seconds, the the pedal is going deeper and softer and I have to push the pedal down some more for the brake to hold and keep the car from rolling. It started doing this symptom this morning and heres the assessments:

1. The fluid reservoir is full which means I am not losing any brake fluid.
2. There are no signs and evidence of any brake fluid leaks on the:
a. All 4 calipers
b. Master cylinder and booster
c. Hard metal lines, rubber hoses, nipples and ferrules.
3. The brake will hold and grab on the first step on the pedal, then fades away when...lets say, waiting for the green light at a stop light.
4. On the first step on the pedal, there is normal resistance to the pedal and then becomes soft and has to be pushed further down to the floor, until it quits holding then have to release and push it again to keep the car at full stop.
5. All non hydraulic parts are in good condition:
a. Rotor discs are good
b. Brake pads are good

Questions:

1. What's wrong with it?
2. If it's the master cylinder, does it fail abruptly like this?
3. I am not convinced that I have air in the calipers because the brake perfect on sudden and quick pushes on the pedal. So what's wrong with it then?

Thanks in advance and any help will be greatly appreciated!

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'85 300D Turbo - CA Version

My money's on a bad master cylinder.

Did you bleed the brakes recently?

Did you jam on the brakes recently?

I'm assuming you don't have ABS.

Sixto
87 300D

sound like a master cylinder. mine had that symptom when i was coming down the hill from the college. Can we all say scary, it was fine and all of a sudden boom, very little brakes, i had to pump them.

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http://i148.photobucket.com/albums/s...titled-1-3.jpg
1981 300d

I agree w/Sixto. The master cylinder can leak internally, with the fluid "recycling" rather than leaking out. One possible cause is bleeding the brakes manually (by pressing on the pedal while an assistant opens and closes the bleed valve). If you press down farther than you normally do while stopping the car, the piston will enter "uncharted territory" in the cylinder. This lower part of the cylinder is rough and will damage the seal on the piston, allowing fluid to slip past -- resulting in exactly the symptoms that you just described.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
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"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I did a brake bleed Nov. of '07, I believe...and what do you mean jam on the brakes? is it like stepped on the brake so hard so it locks?....I think I just did, an hour ago since I'm having these symptoms, so before I drove home, I tried to mash on the brake pedal to see and check if the car is going to stop, in case I need to stop it instantly, to see if it's safe to drive it just to get it home...and it does, it just wouldn't hold for too long, on a stop light...

..and yes Sir, you are correct, it don't have ABS.

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'85 300D Turbo - CA Version

That's what I was thinking when I first felt the problem, the feel was all normal, but not until I hold the pedal, then it fades away, so in my mind it's the master cylinder. Do you know by chance if there's a rebuild or repacking kit available?

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'85 300D Turbo - CA Version

Master cylinder is what my instinct tells me too. Do you think there is a rebuild kit available?

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'85 300D Turbo - CA Version

doubt it. the inner wall is probably scored and just has to be replaced.

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http://i148.photobucket.com/albums/s...titled-1-3.jpg
1981 300d

I had an SD that had a similar problem but found it to be a vaccum leak...the booster was losing pressure everytime I hit the brakes...I had about 2 good stops one after the other then the pedal was really hard.

Are there any other symptoms that suddenly appeared? Check all your vacuum connections and see if one is off or cracked. Also, make sure all fittings are tight from the vacuum pump to the booster.

hmm...you could be right, but here's the deal... I'm going to be on a 500 mile road trip on Sat, but if I order the part, I don't think I'm gonna get the part Mon or Tues the earliest. I hate ot install a non-MB OE, but I think I'll have to bite on what Advance Autoparts has on their website, for $50 something, "Buy OnLine & Pick-Up Today" or probably tomorrow huh? It's a rebuit though, and their rebuild kit is $130+ something..... I also hate to buy 'em from the stealership.....

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'85 300D Turbo - CA Version

They're not all that expensive....and its a VERY critical part....so I would just replace it and be done. If it were me I'd probably get an OEM one from the stealer....

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-diesel is not just a fuel, its a way of life-
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'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

...but the "pedal becomes hard" is not in my symptoms...and I don't believe it has any vacuum leak....tranny shifts silky smooth, all door locks work, engine shuts OFF with the turn of the keys...

..but to tell you honestly, I thought of that earlier this morning on my way to work when the symptoms starts persisting but immediately ruled it out because the pedal was soft as a sponge.....

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'85 300D Turbo - CA Version

Yea! I agree!

What do you think about ATE from the BuyParts tab? Isn't it that ATE is also the OE for brake calipers?

I'll probably give the stealership a ring tomorrow and see if I can make a good-reasonable deal with them?

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'85 300D Turbo - CA Version

Too long ago.

As Jeremy described with bleeding, when you jam on the brakes, you can push the piston into an unused section of the master cylinder. The unused section is subject to whatever moisture there is in the brake fluid and can corrode. That's why MB recommends frequent fluid replacement. The more used section of the master cylinder is continuously 'cleaned' by the piston so corrosion takes a really long time to form.

Sixto
87 300D

ate is oem. I have used phil and received the part the next day but he is in the same state that i live in. But phil is part of fast lane so it shouldn't make a difference give the buy parts tab a try it will most likely be there in time.

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http://i148.photobucket.com/albums/s...titled-1-3.jpg
1981 300d