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Old 12-21-2014, 09:53 AM
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Join Date: Dec 2014
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Stumped on Vacumn Problem

Trying to get my son's 83 300TD going. He bought it about 5 months ago and it's been running perfect until last week, it lost the power brakes and it won't turn off with the key. After reading all the related threads on this site, I did the following testing:

1. Line from vac pump test steady 25 hcg with vac guage.
2. Bypassed pump and hooked up shop vac, problem remains.
3. Isolated shutoff valve and it shuts off with hand vac pump (with very little vac).
4. Isolated the key line and the key shuts off fine.
5. Blocked off everything but the brake booster, and still have no power brakes.

So I think either I've got a bad leak somewhere or maybe the powerbooster is bad. Kind of stuck here, and don't want to just start part swapping.

Any ideas on where to go from here?
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Old 12-21-2014, 10:06 AM
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Location: central Texas
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What exactly do you mean by " shop vac " ?
It sounds like you have a mityvac type of tool...
that should be used for these tests....
instead of ' blocking off'.... can you JUST attach vacuum to the brake booster ?
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Old 12-21-2014, 10:19 AM
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Join Date: Dec 2006
Posts: 3,011
First of all, a Mityvac is indispensible if you own a Mercedes diesel. If you don't have one, get one.

The way to trace a vacuum leak is to follow the vacuum circuit back from the pump, and follow each branch. Every place the line branches, break it apart. Use the Mityvac first to make sure the branch is seeing vacuum from the pump. Then pull a vacuum on the line to see if it holds. If everything is ok, the problem is on the other side of the branch. If not, it's on the side you've opened. Reassemble, and keep going until you've located the problem.
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Old 12-21-2014, 10:27 AM
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Location: Carolina
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Sounds like you have a bad leak at the booster.
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Old 12-21-2014, 10:42 AM
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Join Date: Dec 2014
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Shop vac----just a regular old shop vac that has an attachment that makes it work the same as a mity vac.

Attached vacuum to just brake booster, and still have hard pedal (pumped it up, reaplied vac - pedal drop). However, I disconnected the metering valve form the booster and plugged it. If I block off the two supply lines and put my guage on it, I get 25hcg there still. Once I add either one of the supply sides lines back onto the metering valve, no vacuum. It seems like I'm dealing with 2 major vac leaks at the same time, so right now I'm bypassing the brake booster and concentrating on the key shut off issue

Also, there are two brown lines coming through the firewall, one goes to the (front) shutoff valve, and the other is the ignition supply side. If I understand some of the threads correctly, the one that goes into the front of the shutoff valve is a vent line. Anyway, I used a t-fiting to isolate the ignition supply side, the black line coming out from behind the vacumn shutoff valve, and I blocked off the vent............key shut off car fine.

I guess I need to isolate the hvac line (green i think?) and see if that clears up that side.
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Old 12-21-2014, 12:39 PM
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Join Date: Dec 2014
Posts: 47
Vac line is cheap. Very cheap.

I had some issues in my 1995 E300d with my lines, and so I just bought a roll of both sizes (not sure if they are same for your car) and printed a vacuum diagram.

Start at one end, and replace them all.

Newer vacuum lines, combined with older vacuum lines will just lead to more problems.

It was a 3 hr project, and cost me about $30.

it will also give you the ancillary benefit of knowing for SURE that your lines aren't the issue, and you can more easily pinpoint and solve future problems.
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Old 12-22-2014, 02:13 PM
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Join Date: Dec 2014
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I borrowed a mity vac. So heres the results of my tests with that.

At the vacuum pump, mity vac reads a steady 20.
I disconnected the vacuum meter valve from the brake booster. Wither everything else hooked up, I have no vacuum off the meter valve. I disconnected the ignition vacuum lines and plugged off the connectors. With the ignition lines blocked off mity vac shows 15 off the metering valve.

I hooked the mity vac to the brown ignition line............won't take a vacuum. If I plug the brown/blue line, then it will hold a vacuum. Not really sure what the behavior should be.

So I believe that the vacuum mechanism in the ignition key is bad. Is that repairable? For now, we've run a remote into the interior so he can shut the car off.

My other concern is the vacuum does hold 20, but it seems "sluggish". IE, if it's at 20, and I dump the vacuum, it takes a few seconds to get back up to 20. Not sure how fast the "recovery" time should be.
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Old 12-22-2014, 03:30 PM
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Location: Middle TN
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Your vac pump and shut off are working. Disconnect everything including the brake booster. Plug all of the disconnected lines and measure vacuum. Add & reconnect the booster. You should have vac very close to full vac and your brakes should work normally. If not, fix that 1st.

Track down the vac diagrams for your vehicle so you have an understanding. Generally, one goes to the climate, another to the doors and another to anything the 123 wagon has that a 126 doesn't that I don't know about. A mity vac is big enough to pull vac for all but the brake booster. You know the booster is at fault when you test and have full vac then add the booster only and it doesn't function correctly.

Next, start tracing the other circuits. The parts store has a stepped plastic connector that will allow connections to several sizes of hose. The connector is of poor quality and may need a little epoxy set over night to seal it up. Buy 2. They are cheap and handy. Besides, you'll lose or break one and the spare will save a trip to the store. There is also a kit with different size connectors and Ts etc.
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Old 12-22-2014, 04:10 PM
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Location: OC Cali
Posts: 110
Are the barbs on the brake booster line cracked? damaged?
1985 300d Turbo Diesel
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Old 12-22-2014, 04:26 PM
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The vacuum lines themselves are rarely the problem. It's most often the connectors. The rubber cracks and expands and allows leakage. A very small leak can cause huge problems. That's where I would start.

I generally grab some good connectors when I'm at the junk yard so I've got spares when needed.
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Old 12-22-2014, 04:29 PM
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Location: Sacramento, CA
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The key vacuum switch is a SPDT in electrical parlance. The tube to the shut-down valve on IP is the "common pole". The key switches that tube to either atmosphere (key in "run") or full vacuum (key in "off"). There is only the one "full vacuum" connection to the rest of the system, so easy to isolate. BTW, don't mix up the 2 tubes, it matters.

To test just the booster, disconnect it and run a hose over to the intake manifold of an idling gasoline car - a poor boy's vacuum pump (also good for initial rough vacuum of an AC system, finish w/ hand pump). Test the brakes that way. Another data point is if the gas car runs faster connected to your booster, you know it is leaking bad. If an old non-EFI gas car, it would stumble lean. I have a bag of various size rubber hoses, 1/4" PVC tubing, vacuum T's, etc that makes such connections easy.
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Old 12-22-2014, 04:51 PM
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Seems like you already isolated each possible device attached to shut down and brakes that are linked via vacuum, except the brake booster.

Disconnect the main hose or vacuum line to the brake booster. Test brake booster directly with hand pump (or shop vac). It should hold vacuum. Brake pedal, with vacuum supply to brake booster, should feel soft but not spongy. Immediately after shutoff, the pedal should still be soft, but a few pedal applications and it should stiffen up.

If it is not holding vacuum, your brake booster needs to be replaced. Went through the same ordeal recently.

w123 brake booster problem
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