Just read 43-660. Odd that it describes a special tool needed (measuring connection) that you have to make instead of one you can buy. Maybe that's the reason the vacuum pump is so neglected.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That test on 43-660 reminds me of a drunk test Steve Martin took in one of his movies.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Gents -

Am following this thread with great interest...! My vacuum pump has been on my short list of things to take a look at; it's now moved up.

Curious; Where are you guys downloading the manuals from? For what it's worth, my 617.95 Engine Service Manual references a Section 42-620 (which I don't have!):
Anyone find any definitive sources for some of the parts referenced here - the bearings, the plates, etc.?

By the way, (I forget at the moment), where does the electric vacuum pump - in trunk - fit into the equation?

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'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

The junk in the trunk is for the power locks.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi

Hello, This bearing or bearing cage that we seem to hear about failing after so many miles....Is it in the newer style vacuum pump or the older style pump or both?. We have a 1985 300 TD.

I took apart a spare v.pump last night to get one of those valves to use in the oil drain tube....I have heard they are the same part. Does the little rubber sealing ring go into the oil drain tube or do I take it off ? I don't think that there was any rubber seal ring in there when the old one was removed. I am guessing it is for the vacuum pump application and not the oil drain tube so remove?
Thanks.

Inside each bearing is a plastic separator, called a cage, that holds the six balls and spaces them evenly. It's part of the bearing and not serviceable separately. If the plastic cage disintegrates, it allows the balls to collect on one side, opening enough of a gap in the bearing to allow the bals to fall out.

I suspect all the original W123 vacuum-pumps had plastic bearing-cages. The new replacement pump on my'84 300TD has metal cages, while those on the new lever-kit I installed on my '82 240D, are plastic.

Happy Motoring, Mark

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DrDKW

I find the claim that the vacuum pump shuts off at 25" to be dubious at best.

A strong vacuum signal is an integral part of the braking system on these cars, and if braking uses vacuum, it is logical to conclude that a hard brake would deplete the vacuum significantly and cause a lag in it being restored. During that lag, braking performance would be diminished. As important as braking is to safety, I can't see the engineers designing it this way.

But I could be wrong...

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cbjukraine
'84 300D

Considering how few vacuum pumps will pull 25" Hg, the claim is probably as irrelevant as it is dubious.

25" is impossible at my elevation.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

FSM 43-660 vacuum pump test:

engine at 750 rpm idle, vacuum at brake booster (custom Tee adapter needed) should reach 0.5 bar (14.7 in Hg) in around 15 secs (varies with the booster model). Engine shut off, if vacuum drops more than 0.2 bar (6 in Hg) in 30 seconds, check system for leaks.

If the specified data is not attained or if the vacuum drop is more than 0.2 bar in 30 s, check vacuum system for leaks. If no fault is found, recondition vacuum pump or renew.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I just tested mine where the hard metal line connects to the brake booster line. 24" Hg at idle, so it might get up to 25" at higher RPM...

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cbjukraine
'84 300D

The most the gauge will read in Denver is about 18" due to the altitude since the measured is correlated to atmospheric pressure. Not sure what the pump 'would think' it is. Would it act as if it were measuring 25" at sea level or would it keep thinking it needed to pump more?
The whole issue is more dubious in my mind all the time.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Vacuum at altitude:
Effects of Atmospheric Pressure on Vacuum Level

Note that the Chart is for absolute Vacuum not the max Vacuum you Vacuum Pump can put out. But, you can use the percentage of loss of vacuum to calculate.

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84 300D, 82 Volvo 244Gl Diesel

I've seen contradictory charts about what loss occurs at what altitudes. But if Beagle's claim is correct that it takes 25" of vacuum to park the pump, it couldn't occur above a certain elevation and pumps would be working full time.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

How did ypu test it?
Did you make the special Tee per the FSM?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked