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  #1  
Old 11-23-2007, 04:54 PM
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Brake issue, help!

Ack, I seem to have developed a braking issue! For the past couple of weeks, my brake assist (vacuum booster) would go out, intermittently. When it went out, I would lose the ability to shut off the car as well, and had to use the engine lever.

So far, I've tried the following:
* Replaced check valve at vacuum pump. Old one was practically disintegrated.
* Replaced main vacuum line from pump -> booster (and the two split-offs). This seemed to fix the issues for a while, but the problem came back again last night.
* Bought myself a MityVac, and checked various components.

At first, I thought my pump was shot, but the pump is putting out 25Hg's of vacuum.. both at idle and at 3,000RPM. Pump appears to be fine. I checked the brake booster, pumped it up to 20Hg, and let it sit for about 6 minutes. Over the course of 6 minutes, it only lost 1/2 an Hg of vacuum, which could be attributed to a loose connection on the Mityvac.

This is where things get strange. I checked the main vacuum line at the connection to the brake booster. At first, it only pulled 15hg of vacuum. Suddenly, it fluctuated to 23, back to 15, and seemed to prefer 15HG's or less. This didn't seem right to me... shouldn't the main line be giving the brake booster the full 25hg of the engine? Tomorrow, I'm going to try plugging the other two lines coming off the main vacuum connector, and see if that changes anything.

Do any of you have any idea what may be causing this wacky issue? Is my main vacuum line that I just purchased, defective? I appreciate any help you guys could give me. Thanks!

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]
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  #2  
Old 11-23-2007, 05:03 PM
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I've got no clear idea on what is causing the problem, but if I were you, I'd run a line into the passenger compartment so I could monitor the vacuum as I'm driving and figure out when and how the vacuum is disappearing. It might be relevant if it is disappearing randomly or only when you step on the brakes.
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  #3  
Old 11-23-2007, 09:52 PM
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Quote:
Originally Posted by kerry View Post
I've got no clear idea on what is causing the problem, but if I were you, I'd run a line into the passenger compartment so I could monitor the vacuum as I'm driving and figure out when and how the vacuum is disappearing. It might be relevant if it is disappearing randomly or only when you step on the brakes.
I agree.

It appears to be an intermittent vacuum leak, however, most consumers on the vehicle won't leak enough air to reduce braking effectiveness. The booster, however, is not a device that works on occasion. Either it does or it does not.
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  #4  
Old 11-23-2007, 10:10 PM
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Hmm. The loss of brake power appears to be entirely random. I did a little more tests, and here's what I found:

* Let engine "build up a charge" of vacuum power. Put car in reverse, hit brake pedal. Booster kicks in, and the car stops on a dime. Hit the brakes again, slightly less effective. Third time, and I was operating off manual brakes, no boost.

* Checked vacuum off of the main hard plastic line (Main Vacuum Line?). The "small" split-offs appear to be as follows:
- First split reads 15hg, constant. Engine revving doesn't change anything.
- Second split reads 5-15hg, and seems to vary with engine rpm's.
- "thick" connector to the brake booster itself, reads 15hg sometimes, but will randomly climb to 25hg (which is what my pump put out, when I tested it), and seems to "prefer" 15hg.

I know that the brake booster is probably fine. I don't leak any brake fluid, and when the booster has vacuum built up, it brakes without any issues. I'm wondering if the check valve in the main vacuum line is broken (even though it's a new part), or if my vacuum pump is acting flakey.

Tomorrow, I'll put the mityvac on the vacuum pump, and see if it holds 25hg constantly, or if the vacuum fluctuates. So far, it doesn't appear to fluctuate, and 25hg leads me to believe that the pump was either replaced by the PO, or was recently refurbished, since it seems to put out a good vacuum. Any ideas, with this added info?

Thanks for all the help.
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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]
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  #5  
Old 11-23-2007, 10:27 PM
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Quote:
Originally Posted by Zerohour3k View Post
So far, I've tried the following:
* Replaced check valve at vacuum pump. Old one was practically disintegrated.
Did you make sure that there were no check valve components left behind in the pump inlet? They can shift around and cause intermittent vacuum problems.

Otherwise, the door lock system is a prime suspect. You might try plugging the lock vacuum supply line at the firewall.
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  #6  
Old 11-23-2007, 10:36 PM
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Quote:
Originally Posted by tangofox007 View Post
Did you make sure that there were no check valve components left behind in the pump inlet? They can shift around and cause intermittent vacuum problems.

Otherwise, the door lock system is a prime suspect. You might try plugging the lock vacuum supply line at the firewall.
I never checked for shrapnel. I took the car to a mechanic near where I work, and he replaced the check valve for me, and told me the news about how the thing was disintegrated. I still have to wonder if shrapnel ended up blowing through my system, though. Not really sure if there's a good way to "blow it out", though.. if there IS shrapnel.

I'll try plugging the two split-offs from the main vac line tomorrow (making 100% vacuum go to the booster), and see if that changes anything.
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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]
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  #7  
Old 11-23-2007, 11:02 PM
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Quote:
Originally Posted by Zerohour3k View Post
I still have to wonder if shrapnel ended up blowing through my system, though.
Normally, it's too large to exit the pump inlet. It could probably be fished out with a piece of wire, with the valve is removed.
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  #8  
Old 11-24-2007, 12:47 PM
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You know what the worst part about an intermittant problem is? The fact that I don't know what the heck I did to fix it. I took out the main vac line, and shook it like a british nanny, hoping to see some shrapnel fall out of it. After doing this for a minute or two, I stuck it back onto the car, and did a few more vacuum tests (I wanted to see if the splitter was prioritizing the booster over the "small" splits. It seemed to do this.). Drove around and it worked just fine.

Why oh why couldn't it be an obvious issue? >
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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]
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  #9  
Old 11-24-2007, 01:38 PM
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Quote:
Originally Posted by Zerohour3k View Post
I took the car to a mechanic near where I work, and he replaced the check valve for me, and told me the news about how the thing was disintegrated.
Are we talking about the check valve in the fitting on the pump or the valve in the main vacuum line?

The one that typically comes apart and causes intermittent problems is in the fitting where the hard line attaches to the vacuum pump.
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  #10  
Old 11-24-2007, 03:54 PM
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The mechanic replaced the check valve at the pump.

The valve that I suspect is acting wonky, is the valve in the hard line. I'll have to wait for the problem to happen again, before I can tell for sure.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]
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