I think an intermittent soft pedal is pretty common on a W124. In the last few years, I have replaced all of the calipers, caliper guide pins, master cylinder, and brake hoses. I have manually bled the brakes and power bled them. My problem is that the pedal travel, while not close to dangerous, is annoyingly excessive most of the time. Sometimes, however, the pedal for no particular reason feels great with almost no travel before resistance. I think that I may have a theory for this. Forgive me if someone else has mentioned this before, I did not find anything in all of my searches.

Could the front caliper brackets be the culprit? My original guide pins, as well as the new guide pins, have quite a bit of play in the bracket. I believe that this allows the floating caliper to become misaligned causing the soft pedal because the piston has to travel further to act on the brake pad. The problem is, new brackets are very expensive. I was thinking about trying to find a sleeve of some sort that fits tighter on the pins and that can be driven into the bracket after drilling the holes out to the proper OD of the sleeve. Another option would be to find some less worn brackets at the salvage yard, but if this is a common problem, they may all be like this unless I luck out and find a low mileage set of brackets.

Thoughts?

Tyler

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1985 190D 5 speed - bone stock, 300k+ miles SOLD
1986 300E factory 5 speed - black on black
1987 300D- ALDA removed, timing advanced a bit, R129 rear springs, cabrio front springs, 17" Metris steelies, 250k miles
1997 Dodge Ram 3500 Cummins

Maybe I don't understand your hypothesis but I imagine tired caliper brackets would cause vibration and noise, not or not just increased pedal travel. I think you should revisit bench bleeding the master cylinder and the quality of the replacement. Can a faulty brake booster cause those symptoms?

Sixto
83 300SD

You are right about the noise. I forgot to mention that. There is a pronounced knocking sound when hitting sharp bumps that is not present when the brakes are applied. That noise can be replicated by taking the wheel off and grabbing the caliper and rocking it up and down. The fact that it can move at all is due to the play in the pins and pin bore.

I am confident there is no air in the system.

Failed booster should only cause a hard pedal I would think.

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1985 190D 5 speed - bone stock, 300k+ miles SOLD
1986 300E factory 5 speed - black on black
1987 300D- ALDA removed, timing advanced a bit, R129 rear springs, cabrio front springs, 17" Metris steelies, 250k miles
1997 Dodge Ram 3500 Cummins

Sounds like a failing master cylinder.

Press the pedal down very softly and slowly, does it continue to drop? Now hit it hard and fast, does it stay higher? Leaking piston/seal.

Can you pump the pedal a couple of times and get it to brake higher?

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Gone to the dark side

- Jeff

Definitely not the master cylinder. After the pedal gets to the end of its free travel, it is very firm, not at all like any master cylinder failure I have encountered in the past. I have had this car for 6 years and it has had this problem since I got it. About 5 years ago I replaced the master cylinder with no change. In the last 4 years all 4 calipers have been replaced and braided stainless lines from WRXtra installed. No change at all. I want to reiterate that after this excessive free travel is taken up, the brakes work AWESOME. Better than any vehicle I have ever owned. It is also strange that sometimes, about every 5-10 brake applications, the pedal feels exactly as it should with very little free travel before resistance is felt in the pedal.

The purpose behind this post is twofold. First, to see if anybody else has encountered this issue and resolved the problem with a fix to any play in the caliper brackets. Second, for people out there that have this issue to give some thought to a part that most of us take for granted like the caliper bracket. Like many, over the years I have thrown parts at the problem like the master cylinder and calipers and even the hoses thinking those have to be it. If all along it was worn out caliper brackets, I could have saved some serious money.

Of course another solution would be to bolt on fixed calipers from a later W124 or one of the V8 models, but I have read that none of them would fit behind the CLK wheels I have installed. I would rather stick with the floating caliper setup and make it the best it can be. I might call up a local salvage yard and ask for a couple brackets and take some guide pins with me and see how much play there is before I buy them. I just hope they are not all like this. I might end up putting a sleeve in these after all. I wish new ones weren't $200 a piece.

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1985 190D 5 speed - bone stock, 300k+ miles SOLD
1986 300E factory 5 speed - black on black
1987 300D- ALDA removed, timing advanced a bit, R129 rear springs, cabrio front springs, 17" Metris steelies, 250k miles
1997 Dodge Ram 3500 Cummins

I have never seen a floating caliper anywhere that was supposed to slide any way but in & out. The problem on most is gummed up pins that result in sticking. Fix whatever is causing the poor fitting on the pins. You should not be able to feel sideways play on the pins, only smooth in & out movement.

A failed master can leak slowly internally and still come to a point at the end of it's travel where the pedal is hard. Sometimes the leak is technically external but can only be seen by pulling the master away from the booster.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Yes, I have seen the problem with old grease, corrosion, etc. cause the caliper to not travel freely on dozens of vehicles of all makes. That is not the problem here. The problem is that the bore that the pins ride in has been worn so much the caliper has too much play side to side. I even purchased new pins recently to see if it would help at all. My dial caliper showed that the difference in diameter between new and old pins was less than .002". In other words, the pins were not worn, all of the play is in the brackets which I assume to be original on a car with 245k miles. I through the new pins on just for the heck of it and no change. The bore was as clean as a whistle and new synthetic grease applied to the pins.

The master cylinder is not the issue. I am 100% sure of it. It has been on the vehicle for 5 years and has not gotten any better or any worse than the master cylinder that was on the car when I bought it 6 years ago.

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1985 190D 5 speed - bone stock, 300k+ miles SOLD
1986 300E factory 5 speed - black on black
1987 300D- ALDA removed, timing advanced a bit, R129 rear springs, cabrio front springs, 17" Metris steelies, 250k miles
1997 Dodge Ram 3500 Cummins

Tyler,

I have had the exact same symptoms as yours which developed suddenly 8 years ago. I have replaced the hoses, rotors(discs) and pads all with ATE. I've power bleed it three times using normal Dot 4 and lower viscosity fluids and also bleed the traditional way pushing the pedal.

It is interesting to note that you can have a normal operating pedal occasionally, as I can too. My next step was to rebuild calipers then replace master cylinder but after reading this has made no difference for you, I may as well look elsewhere.

I drove the exact same year (93) W124 just yesterday and that has perfect firm brakes with no where near as much pedal travel as mine. The only obvious difference was I have a Lucas (silver) brand booster and his was a genuine (black) Mercedes booster but I assume the genuine is a Lucas anyway just painted a different colour.

Anyway just wanted to point out same brake 'effect' as yours and unresolved.

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1995 E300 Diesel

Interesting Does your 93 have a floating single piston front caliper? Do you know if the car you drove yesterday has the floating caliper or fixed caliper? I know I read somewhere that someone had a similar problem as this that magically went away when they installed fixed calipers from a 400E as part of an upgrade they were doing anway. That was part of what got me thinking that there might be something there.

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1985 190D 5 speed - bone stock, 300k+ miles SOLD
1986 300E factory 5 speed - black on black
1987 300D- ALDA removed, timing advanced a bit, R129 rear springs, cabrio front springs, 17" Metris steelies, 250k miles
1997 Dodge Ram 3500 Cummins

I still suspect master cylinder, ... the difference would be pushing the pedal very slowly and gently will let it sink, and more quickly will keep it solid (the extra pressure seals the leak that was getting around the piston seal in the master cylinder). It can also have wear only in the normal operating range of the piston (pedal), where the bore beyond that is in better shape and seals better.

I don't really understand the problem however. I have done testing in these cars in the early days, have owned several from new to 250k+ miles, gas/diesel/4matic/wagon/sedan/early-floating/late-4puck and my current one converted from floating to fixed 4puck and have always had solid brakes. However, I do have a nice set of '87 floaters from my 300TD, I could be persuaded to part with them if you find yourself in need.

Look at the pads, you should have reasonably symmetrical wear and parallel to the backing plate. If you see more wear on the piston side, or on the outer/not piston side, this would indicate to me that your caliper is sticking and not sliding, wear top vs bottom of the pad would also indicate that your caliper is not sliding properly.

I like to lightly lubricate the slides with a good high-temp anti-seize, I don't know if it's factory approved, but it has worked for me for years.

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Gone to the dark side

- Jeff

I would tend to agree with you, but the pedal did feel EXACTLY the same before and after the MC swap years ago. Hard to believe that two master cylinders, one with many miles on it and one freshly rebuilt, would both be exhibiting the same symptom. I don't want to make it sound like this issue is any more than an annoyance on an otherwise near perfect car. I have driven many vehicles where the pedal moves much further than this before resistance is felt. It is just that it is so close to perfection, that I want to get that last little bit

My car probably had 2-3 brake jobs done before I bought it. It is possible that lubricating the guide pins was neglected, and thus the guide pin bore was worn excessively.

I might take you up on your offer on the brackets. Would you be able to stick a couple of good guide pins in and see how much wobble there is in them? I can tell you that my current ones, especially the driver side that I hear the knock from (except during braking, even light braking), have A LOT of play.

Thanks!

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1985 190D 5 speed - bone stock, 300k+ miles SOLD
1986 300E factory 5 speed - black on black
1987 300D- ALDA removed, timing advanced a bit, R129 rear springs, cabrio front springs, 17" Metris steelies, 250k miles
1997 Dodge Ram 3500 Cummins

Just my two cents worth. As the caliper moves in ways not intended. The piston is driven back in farther by the rotor. It takes more brake fluid than the design intended before the proper rotor contact is re established.

I think everyone recognized this as a reasonable possibility but just in case.

Now if a steel bushing was available it might be workable just to ream out the original holes to almost the outside diameter of the bushing and drive them in. That bushing should have a fairly small wall thickness though. I am thinking about 1/32 of an inch. Reaming carefully should keep the alignment usable. Hopefully the centering distance between the two holes as well.

Confirmed both single piston floating calipers on the front on both cars and both Lucas brand.

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1995 E300 Diesel

Has anyone suggested the brake booster?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Is there a picture of this bracket?

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC