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  #1  
Old 03-24-2006, 11:29 PM
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Intermittent Vacuum Problem

1982 300D with intermittent vacuum problem. The symptoms are we I'm driving the vehicle the brakes will lose their power assist. This obviously makes braking extremely difficult, but then the assist will return. If this happens when I'm making the final stop where I'll be turning the vehicle off, if the brakes aren't working the car will not shutoff. Then I have to turn off the engine using the stop lever under the hood. I have checked the vacuum for the two lines that branch off the main line to the brake power assist unit. However when I check the main line, the one connected to the brakes, I wasn't getting a good vacuum check. Any ideas what kind of symptoms occur when the brake power assist module fails or begins to fail. Or do I have some kind of intermittent vacuum leak that is affecting the braking assist. I have the link for the vacuum diagnostic and will work through that later this weekend. Just wondering if anybody else has had similar situation and what was wrong in their case.
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  #2  
Old 03-24-2006, 11:35 PM
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In my 1980 300D the vacuum assist (big black part behind master cylinder)
had to be replaced when stepping on brakes they stop being assisted to
actually pushing back against me.
This piece was replaced without disconnecting any brake lines.

If you seem to be losing vacuum and the vacuum pump is not doing its job that is a easy rebuild. But I have read that there is also a one-way valve on the vacuum
pump on this car. But I do not know how to tell if that is bad.
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  #3  
Old 03-25-2006, 11:29 AM
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Quote:
Originally Posted by av8or
I have checked the vacuum for the two lines that branch off the main line to the brake power assist unit. However when I check the main line, the one connected to the brakes, I wasn't getting a good vacuum check.
Your issue is a vacuum issue and not an issue with the brake booster.

Connect up the vacuum gauge to one of those two lines that branch off the main line and get a reading at idle in the driveway. Post the reading here.

If you can run the gauge into the cabin and drive the vehicle while observing the gauge........even better. We're looking for a drop in vacuum that causes you to lose the power assist. It could be the vacuum pump........or it could be a leak.
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Old 03-25-2006, 11:37 AM
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The symptoms described are very consistent with a failure of the check valve on the vacuum pump. It's inside the nipple where the line attaches. If you remove the nipple, you might have broken parts fall out in your hand. Easy fix.
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  #5  
Old 03-25-2006, 07:24 PM
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Okay, finally got a chance to check the vacuum pressure. I check both of the tubes that branch off just in front of the brake booster. Both had a vacuum pressure of 21 psi. The brake pedal felt fine and the car shutoff, so the vacuum was obviously working, this time. What is the next logical step...appreciate your comments and advice.
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Old 03-26-2006, 11:57 AM
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Don't know how this got closed. I guess I did it. If I did then it was purely by accident--I want somebody to help me out.
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  #7  
Old 03-26-2006, 02:37 PM
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Quote:
Originally Posted by av8or
Okay, finally got a chance to check the vacuum pressure. I check both of the tubes that branch off just in front of the brake booster. Both had a vacuum pressure of 21 psi. The brake pedal felt fine and the car shutoff, so the vacuum was obviously working, this time. What is the next logical step...appreciate your comments and advice.

You've confirmed that the vacuum pump is good. Therefore, either you have an intermittent leak (doubtful).......or........the check valve at the vacuum pump discharge port, as TF has noted, is the most likely culprit.
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  #8  
Old 03-26-2006, 04:14 PM
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If you have vacuum at idle then the pump is working, but not under any sort of load. I woiuld do the test and have somone pushing the brake pedal over and over. Then see what happens. The pump may not be able to keep up with the vacuum demand in that situation. Then I would say a pump rebuild is in order.

If you want, you can connect a tee to one of the ports in the main vacuum line and a long vacuum hose with a gauge attached. Then go drive the car and see what the vacuum is doing.
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Old 03-26-2006, 08:54 PM
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Okay, I guess I need a little more specific information to locate the check valve. Exactly where is it. Is it part of the distribution joint that has the two leads off of the main line going to the brake booster or is it somewhere else? There was a mention of vacuum discharge. Is that on the pump itself? Sorry to be clueless here. Thanks for you help.
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Old 03-26-2006, 09:43 PM
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The check valve is in a fitting on the vacuum pump itself. If you remove the vacuum line at the pump, you can then unscrew the check valve with the vacuum pump still attached to the engine. Inside the fitting, there is a small metal retainer which tends to break. When this happens, the check valve will often work some of the time, but not all the time. In any case, it's easy to check.

Looks like this:
http://63.89.49.212/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1PY1E2LZD1RJ1AMPUT&year=1982&make=MB&model=300-DT-001&category=D&part=Vacuum+Pump+Check+Valve

Last edited by tangofox007; 03-26-2006 at 09:48 PM.
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  #11  
Old 03-27-2006, 11:17 PM
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Okay, I removed the metal tubing to inspect the check valve. There does not appear to be any sign of a check valve. The piece connected to the pump has absolutely nothing in it, the same basic diameter as the tubing. Looking into the vacuum pump itself I could not see anything. So now what? Have check valve bits been sucked into the pump and is that causing the intermittent vacuum pressure. Does the check valve reduce the vacuum pressure as well as prevent leak down? If that is the case wouldn't my vacuum pressure be a lot higher than 21 hg? So is the next logical step to replace or rebuild the vacuum pump?
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  #12  
Old 03-28-2006, 05:04 AM
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In a perfect world the vacuum should be 29.99 HG but should be close to 25hg. It may be a wise bet to go ahead and rebuild the vacuum pump, just so you know what you have to start out with.
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1981 300D 147k
1998 VW Jetta Tdi 320k
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1984 300D "Astor" 262k(sold)
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  #13  
Old 03-28-2006, 08:08 AM
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Quote:
Originally Posted by av8or
Have check valve bits been sucked into the pump and is that causing the intermittent vacuum pressure.
Very likely. I would try a magnet or wire to fish the parts out. Failing that, remove the pump from the engine and turn upside down.
Nothing so far suggests the need for a pump overhaul.
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  #14  
Old 04-01-2006, 04:56 PM
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Okay, I have a vacuum pump rebuild kit. Is there supposed to be oil inside the vacuum pump? There is oil inside of everything, even the area in the front of the pump where the 3 check valves are. Okay, I screwed up slightly. I let the single check valve fallout without noting which way it was installed, can somebody help me out. Is there any magic about rebuilding this thing. I'm just taking it apart and replacing the seals the kit has in it---hope I find them all. On the reinstall how do you deal with the spring pressure, or is the low position on the cam easy enough to deal with? Thanks.
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  #15  
Old 04-01-2006, 08:35 PM
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Did you find the remnants of the inline check valve?
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