Ok, so I've read a bunch on here about brakes right now but I can't seem to figure out what my exact problem would be. So a month or two ago, I would once in a while get a brake pedal that goes all the way down to the floor. I didn't lose the ability to brake by any means, I just had to push way lower. I looked this up and it seems this would be the master cylinder failing intermittently. Then, the problem went away. The brakes where fine for these last couple months. Anyways, yesterday, I was driving on a windy road with lots of inclines and declines. At some point 15 miles in, I had to push way far down to brake, just like before, but after another couple minutes, it just became super hard to actually stop the car. I'd be pushing so hard and although the car would eventually come to a stop, it was hard and had there been more cars on the road, dangerous.

Anyways, I stopped the car on the side of the road and it didn't turn off right away, indicating a vacuum leak. Which seemed crazy to me since I recently did redid a lot of vacuum lines including the brake booster line. After reading on here it seems that I might not have a leak, but the car didn't turn off because I probably pumped the brakes, therefore using all the vacuum up.

So... continued driving a couple more minutes. Problem went away. Totally brake pedal was even firm.

Fast forward a couple days of no driving and the same thing happens again on the same road going back home. After 15 miles again. This time I pulled over, braked as hard as I could to stop the car, it smelled pretty bad, turned off the car (turned off right away this time), left it sitting for 5 minutes and started driving again. Brakes were totally normal the rest of the drive home. Firm even.

So.... I just don't understand what my problem is. Vacuum? Master cylinder? Lines? Booster?

Seems like it's not vacuum since the car almost always shuts off right away. Master cylinder seems shot for sure. Wouldn't the problem have to affect all the brakes, so therefore brake lines would mean it's all 4 brake lines? Which would be unlikely, right? Booster? That would mean the brakes are difficult to push, but not falling down to the floor, right?

Thanks,
David

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1985 300D. 179,000 miles.

You're risking your life, and everyone else's around you.

This is not something you want to experiment with, to see if it works later.

TAKE THE CAR TO A MECHANIC WHO KNOWS WHAT HE'S DOING.

That said, I had a hard pedal once. I overshot a stop sign by 15 feet. Had it been earlier, I literally could have killed about 5 school children. My mechanic replaced the power booster. 2 weeks later, same problem. He replaced the line to the booster. 1 week later, same problem.

Someone on this forum told me about the "check valved", located at the top of the radiator. He said it's possible that got a speck of dirt in it or something, and I lost the power brake. I replaced that, never had another problem, but immediately took it to a MB specialist, independant mechanic.

He said MB had a problem that they never owned up to. He feels the problem was that since each time this happened to me was on the first start of the morning, and that I coast down my driveway, then put it in drive and almost coast down to the stop sign, was that the car was not getting any vac build up. He suggested hitting the gas a few times to build up some vacuum.

Like I said, mine was only in the morning. I do think changing the check valve helped. I've never had the problem since. The independant mechanic said since it was an intermittent problem, they'd never be able to make it happen while they're looking at it, so he figured it'd be the lack of vac on a first start of the day. That was 2 years ago or so, and never a problem since I changed the check valve.

But your problem, you're already driving it, and you say your pedal was soft and went to the floor, different situation than what I had.

Good luck, and get the car off the road until it's fixed!

jeff 1991 300d, 147k

Yeah, I'd really of course rather stay off the road until it's fixed. I drove out to the jungle, then I had to drive back. No mechanics or even auto parts stores out there. Hmmm, what is this check valve part number? Or a link on an auto parts store?

EDIT: Does my 1985 model have this check valve? None of the parts websites show it for my model.

Thanks,
David

__________________
1985 300D. 179,000 miles.

I would replace the master cylinder.

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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.

A problem with the check valve would affect the vacuum boost. That does not appear to be your problem.

I third the master cylinder vote.
the hard pedal came from your repeated pedal pushing.
that also used up all your vacuum. causing the no shut off issues.
get a powerbleeder, replace your MC and flush all the fluid out of your lines with the power bleeder. (you can build one from a garden sprayer)
and Yeah, don't drive anywhere until you do this.