Did some brake work on girlfriend's '84 190e today. Sad part is, the brakes aren't any better, just different. Since we've had it, the car has suffered from a stiff pedal with too little travel. Today I rebuilt the front calipers (one was just about frozen) and we're suffering from a low, squishy pedal. The pedal is high and stiff with the car off, and sinks when the car is on. Maybe something to do with the booster? I know the booster is doing something, since the car is incredibly hard to stop with the booster disconnected.
I've bled the brakes a couple times already (and I'll be the first to admit that I'm not great at bleeding brakes). There doesn't seem to be any sign of a leak, the pedal holds steady and doesn't sink down. That seems to rule out a bad master cylinder too, which my buddy swears is the issue. Everything seems to point to more air in the system, though I don't seem to be getting it when bleeding. I'm lost, and all I can think of is to bleed and bleed until the brakes work as they should.


Two points that may be of interest:

With hard braking, the car pulls left some.

I believe the fluid was overheated yesterday. I'm wondering if there's still some burnt up fluid in the system disturbing the whole affair.

I'm open to any suggestions.

I have the same issue and have not found a solution. I've changed the master cylinder, the rubber brake hoses, pads, rotors and pressure bled 3 times. The brakes work ( cars stops straight ), but the pedal travel is too long with vacuum on the booster. I've got a hard firm pedal with no vacuum, soft squishy pedal with vacuum. I know this doesn't help you, but if anyone else has an answer, it'll show up in my "follows" list. I was about ready to have my Mercedes shop change out the booster, but the mechanic there, who has a car just like mine, insists that "in general" Mercedes brakes from the early 90's era are not as "firm" as brakes found in most other vehicles. He drove my car and said my brakes are about the same as his. The car will stop quickly, even activate the ABS, but having to push the pedal half way to the floor before the brakes begin to grab with authority is disconcerting.

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If there is ABS, air can get trapped in the various chambers. A factory service manual would have instructions for bleeding the ABS part of the brake system.

You may get lucky searching google.com for the following words...

mercedes bleed abs

Didn't that start in '85?

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You can't always get what you want, but if you try sometime you just might find you get what you need.

84 is non-ABS.

Did you replace the hoses all around? I hope they are not original...

Are you using a power bleeder or the old fashioned way? I would highly recommend power bleeding. The one time I encountered the symptoms you describe it was due to some air trapped in the rear brake circuit. A good full bleed with a power bleeder cured the issues.

Bleeding with the pedal can also quickly send an old master cylinder to the grave yard....ask me how I know.

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The rear reservoir level is difficult to read. You can really only see it by looking closely at the rear 1/4" of the side the of m/c that faces the engine.

Without knowing this it is very easy to completely drain the rear reservoir, and considerably more bleeding will then be required to purge all the air.

Duke

84 is also the only year that has unvented dinky toy front rotors. This might be a good time to swap in bigger brakes off a later 201 or 124. Even the 284mm sliding caliper W124 brakes are much more powerful than the 262mm rotor.

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CENSORED due to not family friendly words

You won't be able to swap in later style calipers unless you go to a bigger wheel if you are still running 14" original 1984 wheels, they will not fit. Later cars had 15" wheels to accommodate the larger brakes.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I would go ahead and do the MC and hoses at 30 years. My 201's MC was shot at about 12 years. Just bench bleed the MC first to ensure you get fluid out of both ports before moving on to bleed at the calipers.

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Prost!