whunter tested with a reclaimer, and the pump BRAND NEW would only park 2 out of 5 times... an old one would park about the same...

bottom line is this.

unrestricted flow through the pump is bad for it. the pump should only draw vacuum from the factory designed locations, and not through leaks under carpet/door jam/hood, other places with dirty air...
SEAL IT UP. it'll park when it can, but it'll stay CLEAN!~!!

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Any explanation as to why it would only park 2 out of 5 times?

I rigged up another test using the vac pump in my 85 as the vac source. It only pulled 22" Hg, not enough to move and park the vac pump under test. I took a pic of the setup and will post it later. In the mean time I will look for a window air cond compressor for a stronger vac source.

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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

My best guess:
* Valves forming a less than perfect seal.
* Piston seal ring forming a less than perfect seal.


.

Funola
I’m sorry to have to tell you that you are wasting your time with that set up. All you are doing is testing the pump's suction plate valve for leaks. If Whunter was using a piston pump and had even the slightest movement of the rocker then he has proven that the suction plate valve is u/s and leaking. It is impossible to check the piston pump like that unless you draw out of the exhaust valves and block the suction pipe - that is an obvious no-go on a piston pump.

It is however possible to check a diaphragm pump and I checked a very old one I have yesterday by simply connecting the booster connection to the pump exhaust and blocking the suction pipe with my finger. It pulled the roller back 9mm which is not bad for an old relic like that at 2000ft! If there are still any “Doubting Thomas’s” left may I suggest that you try that.

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Beagle

No problem Beagle. I spent maybe 20 minutes on the test and it was not a total waste. In that I did not blow anything up and that it held vacuum indicating the plate valves in both pumps are ok.

Now I am curious how whunter was able to park his NOS piston pump, but only 2 out of 5 times.

Any suggestions on testing a piston pump whether it is parking? What do you think of drilling a hole and get a visual with a borescope? Installing a proximity sensor on the rocker arm to detect movement? Or just listening with a stethoscope? I haven't tried that yet but will do so before doing anything else.

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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

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

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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It does come with one, I didn't take notice of what exactly comes with the pump. I have a new, updated style on the way from fastlane.

Here's my test setup. An old plastic vacuum line connecting the 2 vac pumps.



The wire clamped to the pump is a gauge to indicate if the cam follower moved or not. 22" Hg was insufficient to move it.

__________________
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

The Green Arrow shows direction of the Vacuum Flow. Pulling vacuum on that side will close the Check Valve where the Red Arrow is. That will prevent Vacuum from entering the that Vacuum Pump.

Even after you get past the Check Vlave there is other Valves inside of the Pump that are one way valves like the check Valve.

In fact if you use the Mighty Vac to create the Vacuum it will close both the Check Valve on both Vacuum Pumps.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Thanks D911 for pointing that out. What changes do I have to make in order to run the test? whunter, how did you do your test?

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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

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

It is interesting to see how this thread has developed.

Are most people (or even all?) now convinced that the piston parks?

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I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Good question! Maybe we should post a pole to find out. I would be more interested in how many now plan to replace the bearings and valves.

As far as I am concerned that is a side issue and a No Contest. Whether you believe it parks or believe it runs continuously is irrelevant. The fact is that these pumps are over 30 years old and should have the bearings and valves replaced soonest unless you want to join the thousands around the world who have had them disintegrate. The object of this thread is to try and convince others of that - looks like I have failed.

Over the last 40 years I have seen in the course of my business, at a wild guess, a couple of dozen failures. Certainly many of them were diaphragm bearing failures. I think the reason most of recent failures are piston is that there are now many more piston (post ’79) than diaphragm still on the road. The vast majority were from bearing failure, a few from timer bolt breakage or loosening and others piston jamming (nut coming loose).

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Beagle

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