Hello all, I am usually able to run down problems via the Forum search and using YouTube University, however this time I'm somewhat stumped. I have a vacuum problem and have been running it down.. when I get toward the vacuum pump itself and eliminate all other connections it will pull 22 in of vacuum, the problem is that it takes around 30 seconds to get there. I've looked all over and can't find a method to test the vacuum pump other than how much suction it produces, haven't found any specs on exactly how long it should take to reach its maximum.

I saw somewhere that it should take seven or eight seconds for the full system to come up to full vacuum, as it is now I can't get there at all. I replaced the brake booster with one from the junkyard but that one seems to be worse than the one I took out, I still have the old one and the O-ring looks good visually
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What would cause the vacuum pump to take so long getting to 22 in of vacuum? Is it likely a check valve problem? If I manually shut the engine down after full vacuum is reached it seems to hold pretty well, so I'm stumped.

If I can determine the pump itself is functioning well I can proceed down the line from there.

Any help would be greatly appreciated

I think you've eliminated the check valve being the problem. Put vacuum on the system while it's disconnected from the pump. If it holds then you've got a bad pump.

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84 300SD
85 380SE
83 528e
95 318ic

Thanks for the reply, originally I tested the system one line at a time, then with all accessory lines connected, with pump connected and it held, (barring the CC line which I had blocked off. I couldn't get any joy (well, it held around 14" at most) with the brake booster connected (the original), so went to the boneyard and found another. The new booster isn't doing well at all, even with the old o-ring.

Before I removed the original booster I was doing better. I thought I had the problem nailed down to the booster, but now it seems all tests have regressed. So I went back to the pump to check max suction, it was good at 22" but took awhile to get there. I'm going to do more tests tomorrow and try to get some more detailed and repeatable results.

I'll report back on what I find.

Thanks again

Since it is easy to do you might as well unscrew the check valve from the vacuum pump and take a look. When I did that to mine it had not fallen apart but hosing it out with WD-40 fixed it and that was back before 2010.

If I remember correctly there is also 4 other disc valves inside of the vacuum pump that can get gunked up or have issues.

In the attached picture I believe it is from member Stretch you see gunky oil that got past what is likely a worn piston seal. Stretch has a threads on it with more pictures.

When you can find them the piston seal kits are over $165 each.

When I took my Vacuum Pump apart I sheared off 2 of the little screws. Use PB Blaster or Kroil on the Screws and heat before attempting to remove them. At the time I had a small butane torch but a Candle might do just as well to heat one screw area at a time.

I don't recall shearing off any of the Vacuum Pump to Block screws.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

I'd considered doing that to the check valve, so I will, now that you've mentioned it.. I also have a huge thread bookmarked by Stretch and have been looking for information relevant to my situation.

I had a thought, assuming the check valve holds vacuum as it's produced, which it apparently does, what if the piston seal is slightly compromised and each stroke doesn't produce as much vacuum as it's designed to do per stroke? That would explain the good total vacuum of 22", and also the reason it takes so long to get there, measuring from the output tube with everything else being offline.. I'm going to time the process at idle and report it here.. also revving the engine to 2000-2500 rpm seems to make no difference in the time it takes to build pump suction.

Hope what I'm saying makes sense.

I forgot to mention don't use brake cleaner to hose out the check valve as sometimes it softens plastic or makes it sticky and the valve inside is plastic.

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84 300D, 82 Volvo 244Gl Diesel

Noted.. thank you, I might have done that

Ya, I hosed out one of my Widow Switches with brake cleaner and it got a little sticky afterwards. So now I avoid doing that to plastic parts.

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84 300D, 82 Volvo 244Gl Diesel

Yep, I can see how that would be a problem..

I'll hopefully be back in town this week to do more testing on the vacuum system, my gut's telling me the pump is the problem, but that still needs to be verified of course.

It's a simple thing to remove the check valves and blow through them (or, not) , clean them and then re install .

Once I replaced the balky check valves all my Mercedes get to 20" of vacuum in a matter of seconds ~ like 5 ~ 10 .

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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

Aside from the one mounted where the line comes out, are you talking about the ones inside the case, there are two more?
I need to check out where those are located.. it's probably a good idea to pull the pump for a visual inspection anyway, just need to get the gasket first.

Thanks for the input

I was only talking about the easy to reach external ones because not everyone wants to dig too deeply .

Remember to fully clean each and every allen bolt HOSPITAL CLEAN before fitting a NEW ONLY allen bit into it to loosen them .

I'd not remove the vacuum pump until & unless I had a full rebuild kit with new sealed bearings on hand .

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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

vwnate1, thanks for the recommendations, just got back into town a couple days ago. Mystery solved.. it was the check valve coming out of the vacuum pump, took off the fitting atop the pump and sprayed wd-40 in there.. now the pump is working very well.

I installed a new (junk yard) brake booster while trying to figure it out.. now the booster I put in is hanging up and brakes are not releasing unless I pull off the vacuum line, even then they won't release unless I bleed off some pressure.. all 4 wheels are locking so I'm fairly certain the JY booster is hanging up.

Arrgghhhh.. at least I know what's going on now

Before you change out the booster check the push rod ~ it *must* be loose when the brake pedal is released or the brakes will drag ~ a very common thing after M/C replacement .

Someone here will know how to adjust it, it's usually very simple .

It only needs a tiny bit of free play but if the push rod isn't free to rattle when you shake it bu hand, the brakes will drag .

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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

Thanks Nate! I ended up re-installing the original booster and it's working beautifully.
The other booster was weird, after the car was parked for a few days I went to move it into the garage expecting brakes to drag.. well, they were free and easy so I took it for a test drive keeping close to home just in case. I hit the brakes hard a few times to try and lock them up, they didn't so I shrugged and was returning home driving without touching brakes, they slowly locked up again so I said screw it and changed it out.