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Tue Apr 12, 2022 12:29 pm

So... this may seem like a basic question but can't find the answer even in schematics. The check valve coming off of the main vacuum line which goes into the Y connector for the diffs... which way is the vacuum supposed to stop? Should the air flow INTO or OUT OF the air reservoir? Check valve is number 9 in this linked diagram. Should the air flow go INTO the diff system off of the vacuum pump? That's what I'm assuming. https://www.gowesty.com/images/cms/sized/sized_Difflock%20&%20Decoupler%20Vacuum%20System.jpg

Tue Apr 12, 2022 12:59 pm

Air is pulled out of the reservoir. The check valve prevents it from re-entering.

Tue Apr 12, 2022 1:27 pm

Pchill2 wrote: Air is pulled out of the reservoir. The check valve prevents it from re-entering.

Just because I'm dense... which arrow should air be flowing? Blue or red arrow?

Tue Apr 12, 2022 1:50 pm

The tank holds a reserve of vacuum. The check valve should close when the engine is off retaining vacuum in the tank. If you draw on the valve with your mouth, one way you can suck air. The other you can’t. The end you can’t draw on goes towards the tank.

Wed May 14, 2025 2:15 pm

Does somebody has a picture laying around of the check valve near the reservoir? I don't find the checkvalve on my syncro.

Also, what is nr 5 on the above diagram?

Thu May 15, 2025 12:07 am

StefDS wrote: Does somebody has a picture laying around of the check valve near the reservoir? I don't find the checkvalve on my syncro.

Also, what is nr 5 on the above diagram?

It's an air filter for the atmosphere line.
I've personally never seen it, but saw it for sale which prompted me to look into it.

Sat May 17, 2025 3:27 pm

chrismsnt wrote: Pchill2 wrote: Air is pulled out of the reservoir. The check valve prevents it from re-entering.
Just because I'm dense... which arrow should air be flowing? Blue or red arrow?

Keep in mind; that's an early, pre-production diagram. The production Syncros did not have the Bowden cable (#15), it was part of the "selectable 4x4" system (solid shaft) which VW dropped in favor of the VC (simpler, cheaper, and full time AWD).
I believe #5 air filter was part of that system (lines #16 on page 26-27 below). Stock Syncros don't have. All vacuum lines are driven off the same vacuum source as the power brakes.

Here's an early service manual showing the "selectable 4x4" setup (pages 26, 27, 38, and 40)
Ours shown on pages 34, 35, and 37.
https://www.thesamba.com/vw/archives/manuals/1986_vw_syncro_service_manual.php

StefDS wrote: Does somebody has a picture laying around of the check valve near the reservoir? I don't find the checkvalve on my syncro.
Also, what is nr 5 on the above diagram?
Here's the only pic I have of the check valve area, you can just see it at the end of the Y connector.

Line going to right is connection with vac line to brake booster, bottom line goes up front to diff lock panel, top line to three globe vac reservoir bottle.
All lines are vacuum. Air suction is being drawn into main vac line and into engine intake manifold.
(white lines are for my decoupler)