Hello everyone!

it's been years since I've posted on a forum; used to post up a lot in the Volkswagen forums few years back!

I have a 81' 300TD Turbo, after doing a valve adjustment I noticed that the car was shifting a little lower in the rev range. After doing some research I decided to adjust the bowden cable, now it runs like a champ!

On my test drive, I stopped on a hill and went again. At which point I pressed the brake pedal, and it was rock solid, like the brake booster was no longer functioning. SO I brought it back, and went to shut it off, and it kept running! These two things point me towards a nasty vacuum leak.

I checked over all the obvious vacuum lines, and they appeared in place. So I started the car, and it the brakes were fine, and it shut off reliably for about 15 miles or so, then it decided to repeat the symptoms.

Where should I start looking?

First check to ensure that you have 20-22" of vacuum produced by the vacuum pump. You can check this by hooking up a vacuum gauge or Mityvac to one of the ports on the main vacuum tube going to the brake booster.

If you have good vacuum being produced by the pump, then check the engine shut off valve using the Mityvac to see if it actually works by starting the engine and applying vacuum to the shut off valve using the Mityvac. The engine should shut off immediately. If not, your shut off valve is defective.

If the shut off valve is OK, then you will need to check the many other places for vacuum leaks individually using your Mityvac (door locks, climate control vacuum pods, fuel door vacuum actuator, trunk lock actuator, EGR valve, etc.). Occasionally the vacuum reservoir will also leak at the hose entering the reservoir.

Common question with many posts, so just a quickie answer:

If you measure low vacuum on the main line (post #2), before condemning the vacuum pump, connect a vacuum gage alone to the main tube to the pump, i.e. no other paths that might leak. If you see >15" Hg at idle, your vacuum pump is fine. Then start hooking tubes up and find the leaker. Most common source is the door & trunk lock actuators. There are check valves that are supposed to isolate these leaks from affecting the more critical tranny shift and engine shutdown functions, but they often don't work.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Just to confirm. Is this a 300TD station wagon?

I'll measure the vacuum when I get back home this afternoon!
That's correct, 300TD-turbo

Just to give you an idea, on my wagon I had to redo the vacuum lines, as a few were so loose they would just fall apart while driving around, I had to replace the hissing brake booster, replace the driver's door lock switch, and finally the rear hatch actuator was bad but I haven't replaced it because it's easy enough to do with the key (I have an idea to make one out of a regular door actuator though, since the hatch one is NLA).

The stuff under the hood fixed my random issue of running out of vacuum for shifting and shutoff, so that's where I would start. The rest was a door lock issue, but since they're separated I would leave that alone unless everything else checks out (because it's a pain to go through).

-Rog

Should the brake booster be able to hold vacuum when the Mityvac is hooked up directly to it?


Once the car was showing its symptoms; I pulled a line off of one of the T-Fittings and hooked my Mityvac up, and blocked off the other T-Fitting. The gauge was showing right about the ZERO vacuum area.

I then took the main line off, and hooked my Mityvac up to the hard-line coming off of the vacuum pump. I turned the car back on like this, and the gauge read 20" of vacuum.

I tried to hand pump the brake booster, but wasn't able to build any vacuum.

When the car lost power brakes again, I pulled over and checked at the hard line going into the brake booster. There was zero vacuum, but it was building very slowly. I decided to measure again at the vacuum pump, and it also build pressure from zero but did it much much quicker.

Once it got back up to 20", I re-attached the main line, and re-attached all the hoses, and the car functioned correctly again.

Thoughts?

just from a test i did a few days ago, if you have a good seal on your mityvac at the booster, you should be able to pump down to around at least 15" hg in about two minutes. it gets really difficult to pull anymore than that with your hand pump. it should either hold that level or very very slowly bleed off. i'm having a slightly hard time parsing what you said, but if you directly connected the mityvac to the fitting at the booster then this is what you should have seen.

that said... it sounds like you have other problems as well. vacuum from the motor drivin pump shouldn't "slowly" build up. it should almost instantaneously build back up to 20-24" when cleared to 0". it sounds like you may have a fault in the booster hose, or one of the check valves in the hose, or with the check valve on the vacuum pump, or with the pump itself.

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1981 NA 300D 310k miles

The vacuum you see produced by your Mityvac at the brake booster should hold steady for quite awhile (at least a few hours) with other vacuum ports capped off. The brake booster should hold vacuum after the engine is shut off in order to provide 3-4 vacuum assisted applications of the brake pedal after the engine is shut off. By the way another easy test is to apply the brake pedal after the engine has shut off; if the pedal is hard on the first 1 or 2 tries then the booster has failed.

I think you alluded to this: IF you have a large vacuum leak down stream, your vacuum gauge may read 0 even when measuring at the main line. I disconnect all possible leak sources and plug the ports with plastic golf tees. (Wooden Ts get stuck in rubber.) You may find several small leaks in the branches going to the various cabin systems.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I think I found the problem!

That is the inlet check valve to the vacuum pump. It doesn't come w/ a new pump, and people ask ~$75 for a used one on ebay. I don't think it would stop the vac pump from totally working since there are other series check valves inside, but it surely increases the performance. I would be tempted to try installing a cheap generic check valve in the main tube, though you still need that part as an adapter fitting, even if gutted.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I think that there are some issues happening on the inside of the vacuum pump, next step is taking the pump off and inspecting the internals. Plus, there are some pieces to the check valve that aren't accounted for!

I have a local guy saying he gets some rebuild kits from Mercedes for around $71(his cost) that include the check valve. I kept repeating that this is a piston style vacuum pump(1981) and not a diaphragm pump, but insisted it was correct. Does this sound right?

I ended up calling MB and getting three different part number for the a "vacuum repair kit" before them hanging up on me 😂

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That check valve does come with a new pump.
A new check valve alone is about $85.
Quite a bit to spend if you find that you need to replace the pump in the end.

The last time I checked, the repair kits from Mercedes were very expensive and may be no longer available. Installing the 2 kits would cost considerably more than a new pump.
They may be available from other sources for a better price.

How does this look?




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