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300 diesel brake failure. FYI please read.
(Updated May 7, 2007) Ongoing story about brake failure on my W124. About 6 months ago in the morning, idled back out of the driveway, then went 4 houses down the street to the stop sign, and .... hard brake pedal. Almost like having
NO BRAKES! Some people wrote in that with a hard pedal, you just press down harder, but .... no, doesn't work that way. I'm telling you from experience, I had NEARLY NO BRAKES, REGARDLESS of how hard I pushed on the pedal; it was just hard. I could have killed some kid! Read on and please learn from my experience! I immediately took it back home, checked for fluid leaks, no leaks, and the brakes worked fine. Freaked out, took it to a general mechanic. He figured it was the booster, replaced it for $350. A few weeks later, same problem. Took it back, he replaced the booster hose that leads to the vac pump. 2 months later, same problem. Someone on this forum said could be the check valve. I bought one but hadn't replaced it yet, when it happened again. Same thing. Hard pedal. Always in the morning, after the first start of the day, basically just idling it back out the driveway (not hitting the gas), and after hitting the brakes lightly a few times, then suddenly the next time, whammo! Hard pedal. Took it to a MB specialist shop this time. They checked out the vac pump, kept the car overnight, said it's perfectly fine, at least during their tests. At once every 2 months, it's too intermittent of a problem to find it for sure. They told me MB diesel cars have a problem on first start ups on older models, that MB hasn't owned up to. He said to rev the engine a few times after start up, that will produce more vacuum. He said unlke gas cars, revving will increase vacuum, on gas cars, it reduces vacuum. So FYI, if you find yourself with a hard pedal and can't stop, slip it into neutral, and rev it. That is, if you haven't gone through a brick wall at that point! Other diagnosis from members on the forum were that the brake lines might be needing replacing. The mechanic said if that were the case, I'd be having the problem all the time, not intermittently. Today I replaced my check valve. How do you know if they're defective? Someone said if it rattles, it's defective, someone else said if you can blow through it in both directions, it's defective. Mine doesn't rattle, and I couldn't blow through either way that I could tell. Hopefully the new check valve will have fixed the problem, but I don't know. Thanks, Jeff 1991 300d, 105k Last edited by jbach36; 05-07-2007 at 08:06 PM. Reason: Updating |
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Answer:
There are three logical possible causes at this point:
* Failed vacuum pump valve. * Failed vacuum pump. * Collapsed brake hoses. You need to check the vacuum with a gauge. T into the line and see what it reads. If the vacuum fluctuates wildly = bad valve. If the vacuum is low or zero = bad vacuum pump. If the vacuum is good (20 +) and steady = bad brake hoses. D3012-25391 Vacuum Pump Check Valve IN STOCK PART NUMBER SEARCH RESULTS WITH PRICES Vehicle 1991 Mercedes Benz 300D Turbo 2.5 Part: Diesel Vacuum Pump D3000-99491 Diesel Vacuum Pump IN STOCK D3000-56042 Diesel Vacuum Pump Pierburg IN STOCK PART NUMBER SEARCH RESULTS WITH PRICES Vehicle 1991 Mercedes Benz 300D Turbo 2.5 Part: Brake Hose Note: Front 36.5 cm in length : male and female connector. N7000-26865 Brake Hose Corteco/Freudenberg IN STOCK PART NUMBER SEARCH RESULTS WITH PRICES Vehicle 1991 Mercedes Benz 300D Turbo 2.5 Part: Brake Hose Note: Rear 30.5 cm in length : male & female connectors N7000-26863 Brake Hose PBR IN STOCK Have a great day.
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ASE Master Mechanic asemastermechanic@juno.com Prototype R&D/testing: Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician. Noise Vibration and Harshness (NVH). Dynamometer. Heat exchanger durability. HV-A/C Climate Control. Vehicle build. Fleet Durability Technical Quality Auditor. Automotive Technical Writer 1985 300SD 1983 300D 1984 190D 2003 Volvo V70 2002 Honda Civic https://www.boldegoist.com/ |
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Brake hose
The test they did at the shop was that the vac pump was all within tolerances. They thought it's possible it'd be a check valve as well, but with it being very intermittent (happens once every 2 months), they can't really test it as such.
What do you think about the check valve? Jeff |
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Answer
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Brake lines needing replaced?
When you say the brake lines might need replacing, what happens to them? Do they dryrot, or get clogged, or what? And what behaviour happens when they need to be replaced? Do they eventually leak, or pop, etc?
Thanks, Jeff |
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They usually don't leak or crack..........from the outside they look fine. I don't believe that brake hoses are your problem for the simple reason that you'd have to have four brake hoses with the identical problem..........intermittently. That chance is zero. The check valve for the vacuum pump appears to be the most likely culprit. I'll bet that you lose vacuum to the booster on occasion. You can prove this by connecting a vacuum gauge to the booster line and observing it when the "incident" returns. |
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Since it is only $35 for the valve, I would try that. I'm not a huge fan of intermittent brakes though.
Chris
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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous “Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss |
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I'm guessing vacuum, but without vacuum it should still stop, just takes more pressure, so maybe something mechanical is getting in the way of the brake pedal?
As far as it being a diesel thing, one of my '87 300Ds wasn't started for over three months, I started it this spring and backed it out, had brakes from the moment I put it in reverse to back out, I've never heard of this being a diesel issue, the guy has his head up his tailpipe.
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Gone to the dark side - Jeff |
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My W124 300D is doing the same thing, but it has excellent vacuum, hell the locks will work after sitting 4 or 5 days and me unlocking 2 or 3 times durring that time. On mine if I let it sit and run about 30 to 45 seconds brakes work fine, if not I will be dropping it into drive and digging a hole in the gravel driveway. Once the car has been running a bit the brakes start working fine and there is no further problem with the brakes.
I figured it was the vacuum check valve on mine and was going to order one in the near future.
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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride. '13 Chrysler 200 Touring Candy Red. Grandma's ride. Age and cunning will always over come youth and vigor. |
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I don't want this to come across the wrong way (after reading it it sounds a little snooty , but not its not meant to be) In a 124 series 300D, your locks work off of an electrical vacuum pump and are not connected to the engine's vac system. When you turn the key in the door, it is actuating an electrical vac pump just for the locks.
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Dean |
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How do you know if the check valve is faulty, and ..
How do you know if the check valve is faulty, and what is its function? If I understood its function, maybe I would understand what is causing the problem. I do have the new check valve, I just hate to replace all the parts to find out what the problem is. So far, the boosters been replaced, the hose to the booster, the pads, the rotors, and the vac's been tested. I'm out $800 and still have the same problem (though the rotors and pads did need to be done).
The problem again, is a hard pedal, in the morning, after a cold start, after I idle out the level driveway, hit the brakes a few times, then at the stop sign 4houses away, hard pedal. No other time. That's why I figured maybe the vacuum hadn't gone up to snuff that early in the morning. Jeff |
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Just placed the order, only thing that ruffles my feathers is $6.62 shipping for UPS of a 7 oz object. Bubble rap envelope & USPS would have it here just as quick for less then a dollar.
__________________
'10 Chrysler T&C Stow-N-Go White. Grandpa's ride. '13 Chrysler 200 Touring Candy Red. Grandma's ride. Age and cunning will always over come youth and vigor. |
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Jeff:
The check valve keeps the vacuum in the brake booster regardless of whether the engine is running or whether other items need vacuum. Blow through it in the direction of the arrow, air should flow freely. Blow the other way, no air should flow. Oldnavy: I ship UPS daily, to the tune of around $500/week. They charge a lot, many of my shipments are a 6x9 padded envelope, and it often costs in the $7 range even with my discount, don't blame the vendor.
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Gone to the dark side - Jeff |
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But nothing stops you from USPS small stuff like this does it? Yes I would ***** if they hung me with a new rope.
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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride. '13 Chrysler 200 Touring Candy Red. Grandma's ride. Age and cunning will always over come youth and vigor. |
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