He replaced the booster. It's probably a tough diagnosis anyways, because the brakes failed just that one time. But one time is all you need to go off a cliff. So they worked fine afterwards, and it was probably difficult for him to tell or to see exactly what the problem was. But he surmised it was probably the booster failing, so he replaced it. He thought the vacuum hose was fine. If it fails again, then I'll know it's something else. If I'm not dead, that is.

Mercedes' are designed to crash into things

If the brakes were fine after that one time, what concluded that the booster was the issue ? since the before and after replacement of booster gave same results. meaning brakes were working fine...

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Whether you think you can or cannot, Either way you're right!. by Henry Ford.

If it was the booster your brakes were fine, they just needed some grunt.

Driving non assist brakes you have to put your foot in them.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

It does freak a lot of people out though, with good reason. The family of someone I used to go to HS with junked their late 70's 300D because it had a vacuum problem (maybe the pump was bad). He freaked out just like the poster did when his brakes lost assist.

My last summer job though I drove an old Ford truck around with no power steering and no power brakes, kind of makes you appreciate the things you take for granted in a car.

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1985 CA 300D Turbo , 213K mi

Either people are to young to remember, or don't know whatever. Old cars had no power steering, no power brakes ect. You had to put some effort into it!

If I remember correctly MB actualy publishes two stopping distences in the manual. One with power assist, and one if you lose it. At least in my W126's manual.

I was born in 1985 and have driven a couple of cars with no power anything, MG.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

To answer this question, I really don't know. My mechanic, who is a general mechanic, not a MB specialist, just said it could have been one of 2 things. One was the single line coming in, the vacuum hose or whatever it is. The next was the booster. He drove it prior to working on it, looked at it, and right or wrong, concluded it was PROBABLY the booster going bad. I dunno, I just took his advice and let him do the repair.

I have noticed one thing before, and after this repair, that I didn't really notice until just today. This is an FYI if this happens to you.....

Prior to this, I was noticing while stop lights, that if I was looking around or watching the scenery, that sometimes I would start drifting forward. I didn't think anything of it prior to this, just that maybe I was lax in pushing the pedal down while daydreaming at a stop light. But today, after the repair, I noticed that when I was stopped, the car didn't drift forward.

jeff

Eons ago, my sister had a '68 Chevelle. It had no power assist brakes. It's been years since I've driven a car without power assist, but they will stop with SOME pressure. My brakes were different. The pedal was just hard, meaning it wasn't going down even though I was pressing harder. It was eventually stopping, but certainly not fast enough. It is possible that if I were to drive around without power assist (which I'm not going to) that maybe I would get used to it, and be able to judge when to hit them in order to stop where need be. But based on that one, short incident, I pushed harder and the brakes weren't going down any more than they already did.

Jeff

You are right, unassisted brakes have optimized master cylinders and the brake pedals are longer so you get more leverage. But in a pinch I have been able to stop my 300D when it lost power brakes (supply hose was loose). The W124 may have a bigger MC bore and multi piston calipers which I can't see helping you stop the thing!

How much can you leg press? Didn't mean to talk down to you, I'm but a 19 year old but most of my peers just don't know/care about old cars.

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1985 CA 300D Turbo , 213K mi

I bet you have the problem nailed. I had a similar failure on my 1987 300TD shortly after purchasing it in 1999. Some days the power brakes would disappear. But only on the 2nd or 3rd stop of the morning, and only that one time each day. The brakes operated normally at all other times.

I replaced both the booster and the large vacuum line which feeds it. Problem was resolved, never to reappear. To this day I don't know the root cause of such strange operation.

- JimY

Just yesterday, the problem reappeared; hard brakes. This time I was more ready for it, and thank god, I was just drifting backwards out of the driveway. I really jammed on them, the car stopped. Eventually the brakes became normal. Took them back to the same mechanic, he thought maybe it's the line that comes up to them, said it has a valve in it that could be getting stuck. He replaced the hose, and hopefully that will be the last of it.

What is the deal with the vacuum pump? Where is that located, and how do you fix it or is it just a replace item?

Thanks,

Jeff

I posted 2 weeks ago about brake failure on my 1991 300d. Going towards a stop sign, I hit the brakes, and they were hard ... and weren't stopping me, much. I overshot the stop sign by 8 feet. I could have killed someone or even myself!

A mechanic replaced the power booster, thought that might be the problem. Then yesterday, same problem. Took it back to the mechanic, he thought maybe it was the hose that leads into the booster, and replaced that.

Someone posted to my previous thread that it could have been the vacuum pump. Where is that, how to fix, and is it fixable or just replace?

Thanks,

Jeff

i wouldnt go to the same mechanic for this issue.
spend the hour's labor at the dealer. let this get fixed once and for all. nothing u want to take a chance with.
keep us updated.

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Whether you think you can or cannot, Either way you're right!. by Henry Ford.

The vacuum pump is on the front of the engine, it either fails or it doesn't.

If you are going to throw some more parts at it I would replace the master cylinder and all the soft brake lines. They are getting old, not a bad idea to change them.

If your mechanic knows about these engines he can test the pump, its rather easy and takes a couple of minutes. Along with the vacuum line, they either leak or the don't.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I think there's more to this - and it hasn't been discussed yet here...

First off, my main car (testbed, laboratory, etc.) is a diesel 126, so don't know how much of this applies to your 124. Given that caveat, here goes:

My vacuum pump has never failed, but I understand it may go like this if it ever does: As they start to go, they are known to start dumping engine oil into the brake boost line (the big plastic pipe between the pump and booster body). Because this line has ports which supply vacuum to the shutoff system, EGR, climate control, etc, any number of symptoms may result as this oil creeps into vacuum lines. It isn't hard to imagine that one of these might be an intermittent loss of brake boost. A very common effect is loss of the vacuum shutoff function.

In my case, a small occlusion of 'crud' in one of these ports completely blocked the vacuum shutoff circuit. After cleaning the port, problem solved!

Before getting too crazy with replacing everything: spend 15 minutes, remove this vacuum line, and be sure it's clear.

Lemme know! Lou