You need to connect your Vacuum Gauge directly to the Check Valve and find some way to depress the Lever/roller on the Vacuum Pump. That will cause the Piston to move and discharge the Air inside of the Pump.

When the Lever/Roller is released it will create a Vacuum.

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84 300D, 82 Volvo 244Gl Diesel

If the Piston does not park I believe that when there is a Vacuum in the Pump at the very least it makes it easier for the Lever/Roller to move.

That would decrease the amount of force needed to move the Lever/Roller and decrease the stress and wear on it.

That would also mean the Lever/Roller would not be pushing as hard on the Timer.

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84 300D, 82 Volvo 244Gl Diesel

There is no Metal Piston Ring in the Vacuum Pump.
The head of the Piston has a Ring Groove for an O-ring and a what looks like Nylon Plastic Sleeve gones over that.
The Nylon Sleeve is retained on the Piston Head and moves with the Piston in the (Steel Insert) Bore of the Pump.

Item 28 is the Plastic Seal
Item 29 is the O-ring.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

The piston ring/seals look like a large rubber band and large O-ring.

The vacuum pump piston bore has a steel sleeve, when it becomes scored it must be replaced or junk the pump body.


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If your inlet nipple check valve was intact and functioning properly, you might want to rethink your test procedure.

The steel sleeve is hardened and ground. I have never seen a bore scored - a very hard particle >60Rockwell C would have to find its way in there to do so and it's no mystery how it does! Cover the pump check valve with a cap immediately when you remove the vac hose. It's usually the 2mm thick Teflon band that gets scored. The O-ring keeps the Teflon band pressed against the bore. If undamaged it is a very good seal.

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Beagle

This topic is interesting, it is making me reevaluate my understanding of the piston vacuum pump + have fun testing the cases of used pumps on hand.


FYI:
Capped - sealed the vacuum supply fitting.
Mounted the body to plywood in a vise.
I manually pumped the lever many times.

Later I made a reinforced fitting and glued it to the exhaust valves, attached first one - then two refrigeration compressors on the suction side.

The test results where generally the same:

* The vacuum pump parked any time there was a perfect seal.

* Anything less than perfect = failure to park or in several cases an audible leak.

* Many of the used pumps failed to reach 13 inches mercury vacuum, this is why they where in my bin of cores to be rebuilt.

* Several of my "good used" units failed this test because the valves or piston where leaking, 18 - 20 inches mercury vacuum, but would not hold for any measurable time.

Conclusion:
#1. If the total vacuum system (including pump) is PERFECT, the pump will park.
#2. The MB engineers where smart in designing for the LESS than perfect REAL world conditions.
#3. The design is incredibly robust as proven by durability.
#4. Ignoring vacuum leaks is eventually lethal to the vacuum pump.

My view point:
Unfortunately too many owners overlook or ignore a weak vacuum pump until it fails.
A repair kit or NEW vacuum pump is less expensive than an engine or funeral.

DANGER:
Low or bad vacuum supply is a SAFETY issue because it directly effects your BRAKES.

The first step in any/all vacuum system diagnosis is: Verify the vacuum source, everything else is a waste of time until this is done.


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ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

From my experience in the field, it is not unusual for the steel sleeve to be scored or damaged by sand, dust, dirt over time.

I will try to find time to strip and take photos of some damaged sleeves.


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FWIW at this point, the VP in my friends 240 was a diaphragm type and did lose its balls while he had the car and even though they made their way to the oil pump screen no damage was done to the engine. Lucky guy

Thank you to all you scientific minded experimenters, good info.

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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"

The manual transmission MB vans only have one vacuum feature--the shut off valve. I wonder if the pumps last longer on the vans.

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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

On auto transmission vehicles, the vacuum control valve guarantees that the system is never PERFECT.

Coming in late to the thread, but:

I'm planning on rebuilding my vacuum pump as a preventative measure. Although I can find the check valve and diaphragm kits, I don't see any listed parts for the bearing.

Does anyone know what bearing is needed for a W115?

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1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.

Call Phil at the parts dept here, prices are good and the folks are friendly and helpful.


(888) 333-4642 (Toll-Free)

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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"

If you want only the bearing, read this post, we only sell the arm assembly.
VACUUM PUMP FAILURE! Are you neglecting yours??


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__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

I am trying to find just the bearing. So far I have been able to find some decent quality Nachi (Japan) bearings on ebay for under $10, but I'm not certain they are

I just used a Nachi to fix an idler pulley bearing on my Jetta and it was nice

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)