The vacuum on my 84 300td is intermittent. Usually it works fine, power brakes work, engine shutoff works, etc. But sometimes I'll be driving along and all of a sudden I have no power brakes. I'll get in the habit of STANDING on the brake pedal, then randomly the vacuum will come back and I lock the tires up.
This is obviously not safe...


I have never been at a good place to stop when this happens. However when it's working fine, I can remove the hose from the vacuum pump and put a manual gauge in it. It almost immediately jumps up to ~27 in/hg (it's a harbor freight tool, so probably not too accurate). If I put the pump on the hose going into the car's system, I can pump the system up by hand and it holds vacuum just fine.


If I shut the engine off and put the vacuum pump toward the engine driven pump, I can pump up a vacuum and it holds without leaking down (so the check valve seems ok).


Is it possible the checkvalve is sometimes sticking open? Something else??

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Sounds like the vacuum pump is OK, but you have a massive leak somewhere and I think it might be in the vacuum brake unit. But with vacuum, every thing is suspect until you find the problem maker. A vacuum gauge is pretty low cost and is really a wonderful tool to find the answer to your problem.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

I have a handheld vacuum pump with a gauge on it. I checked all the circuits and they hold vacuum. I even capped off all the vacuum lines except the one straight into the booster and was able to experience the problem.


Anyway, a leak in the hvac circuit or something doesn't leak enough vacuum to kill the brakes. This has to either be in the booster or in the pump. The vacuum hose from the pump to the booster isn't the original that's prone to cracking, it must have been replaced some time before my ownership, it's black, flexible, and not cracked.


But yea, I guess I'm gonna have to just throw a vac gauge on the dash until I can get the problem to happen (and preferably it happens without me rear ending someone, etc...). What a miserable problem to diagnose.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Unscrew the Check Valve on the Vacuum Pump itself and see if it is full of gunk or falling apart inside. Don't use brake cleaner to clean it out as brake cleaner softens some plastic parts. I used WD-40 with no issues.

Mover the Master Cylinder forward a if you can and see if the O-ring is in good shape and dip something inside of the Brake Booster end and see if it comes back Wet from brake fluid.

If you don't see anything obvious with the Master Cylinder O-ring degrease both sides with Brake Cleaner and carefully apply a layer of silicon sealant on the O-ring and install the Master Cylinder and wait at least 4 hours for the silicon sealant to cure and drive the car and see if the trouble goes away. If it goes away you need to replace the O-ring.

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84 300D, 82 Volvo 244Gl Diesel

Odds are outstanding that the inlet check valve has come apart internally. The parts rattle around causing the intermittent vacuum loss.

I am giving 20:1 odds that the diagnosis-related misery will end when you remove the check valve.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Thanks for the good advise. I'm on the road for work for the next few days, so hopefully Wednesday or Thursday night I'll have a few minutes to go in the garage and poke at it.
Check valves are cheap and look pretty easy to replace, let's hope it's that!

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

OM617 Vacuum Pump Check Valve

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

The vacuum "check valve", being discussed here, is around $80.00. Not "cheap", IMHO. It'll be obvious, when you look at it, if it's toast or not.
I'm with "Diesel911" on the opinion of the brake booster o-ring. That part is about 3 bucks. All I would add is, if you're taking the master cylinder off of the booster (to check for a brake fluid leak there), get the new 0-ring beforehand and just put a new one on, while you're "in there". (yours could be 35 years old) Keep in mind, no need to disconnect the brake lines at the master cylinder. Just crack the flare nuts a quarter turn or so, and that'll give you enough movement to pull it forward to check the o-ring (after unbolting from the booster, of course) and install the new one.
I had the same conditions you describe with my '83 240D.Checked everything, like you, and was expecting the worst (bad brake booster, bad vacuum pump, etc). Imagine my delight when it turned out to be the least expensive part in the entire system. My booster o-ring was MIA. Not dry, cracked and deteriorated....it was GONE! Popped in the new one and everything was AOK. Brakes worked normally. Good luck!

Did you notice that 911's first recommendation involved the check valve?

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Yes. What's your point?

Not lot of areas to dig into for finding gross vacuum leaks.
If the vac pump check valve is ok move onto the brake booster and the line.

I once had such a significant leak of fluid into my booster that the internal diaphragm was completely ruined. Into corners, my brakes would get hard and then soft. Very terrifying.

Replacing the booster and MC fixed this.

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-Typos courtesy of my mobile phone.

I was speaking of the check valve that screws into the vacuum pump. Red circle in attached picture. It looks like a fitting but has a valve built into it.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Post #3 mentions a flexible hose instead of the proper hard plastic primary vacuum pipe, maybe it's collapsing and restricting the vacuum ? .

Normally I'd say check valve failing but the hose sounds suspect too .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Yup i know. Just throwing more potential ideas onto the brainstorm bonfire.

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-Typos courtesy of my mobile phone.

I knew what everyone meant by the check valve. Also my vacuum line is hard plastic, it's just flexible hard plastic. Not the original one that cracks the second you touch it.

Anyway, I pulled the check valve and ended up with a threaded tube... Spent 20 minutes fishing with a piece of bailing wire bent into a hook, and ended catching this haul:


Thanks for all the good suggestions, I didn't really feel like getting into that black grimy mess at the front of the engine, but I found the problem...

Now where to buy one? I can't find it on pelican, eeurparts, fcpauto, etc.
I can only find it on the infamous source of mercedes parts and I'd rather not go there unless I absolutely have to...

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...