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  #1  
Old 05-06-2007, 12:48 AM
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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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  #2  
Old 05-06-2007, 03:35 AM
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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




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Old 05-06-2007, 02:50 PM
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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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Old 05-06-2007, 05:37 PM
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Answer

Quote:
Originally Posted by jbach36 View Post
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
Replace it...
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Old 05-06-2007, 06:07 PM
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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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Old 05-06-2007, 06:14 PM
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Originally Posted by jbach36 View Post
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
He's referring to collapsed brake hoses. They deteriorate on the inside of the hose and can stop the flow of fluid to or from the calipers. Usually, they are OK on the pressure side of the equation.........fluid is forced past the partial obstruction........but not OK on the return side.........fluid cannot get back out of the caliper. The indication is brakes that won't release.

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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Old 05-06-2007, 06:23 PM
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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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Old 05-06-2007, 06:58 PM
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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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Old 05-06-2007, 08:42 PM
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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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Old 05-07-2007, 12:49 AM
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Originally Posted by oldnavy View Post
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.
FYI
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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Old 05-07-2007, 01:10 AM
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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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Old 05-07-2007, 07:34 AM
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Quote:
Originally Posted by deanot442 View Post
FYI
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.
Well I certainlly don't look at it as being snooty or snobby, just good info if it's true. I had often wondered in the past why MB did not do this for the door locks, but just figured it was not cost effective is why. Guess I will get a check valve on order today from Roy & Phil.

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.
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Old 05-07-2007, 08:42 AM
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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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Old 05-07-2007, 09:23 AM
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Originally Posted by jbach36 View Post
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
It would be of significant benefit if you could connect up a vacuum gauge to the booster line and observe it in the morning after the cold start and idle out of the driveway and over to the stop sign.
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Old 05-07-2007, 10:39 AM
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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.
And you want me to ***** to to someone that could give a rip I'm alive?

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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