This is what each line does:

one that runs to the center of the driver door switch (plain yellow) - This is the main vacuum supply to the lock system. It should always have vacuum from the vacuum pump (engine running) or the vacuum tank (not running). This line comes from the yellow check valve under the hood.

one that seems to run straight to the vacuum tank (with the blue stripe) - This line just runs to the vacuum tank to charge it when the engine is running and to provide vacuum to the door lock system when the engine is not running. It is connected to the yellow check valve under the hood, right next to the plain yellow line.

There are also the red and green lines that run through the doors. I'm still somewhat unclear on what each line does. - These lines (yellow/green and yellow/red) run from the driver door switch to each of the 3 doors, the fuel door and the trunk lock. The yellow/green lines unlock everything, and the yellow/red lines lock everything. If you pull a vacuum on the yellow/green line at the driver door switch (and vent the yellow/red line) all the doors should unlock, and it should hold vacuum. If you pull a vacuum on the yellow/red line at the driver door switch (and vent the yellow/green line) all the doors should lock, and it should hold vacuum. That is what the driver door switch does it applies vacuum to either the open or close line while venting the other line. If either or both of these do not work (or do not hold vacuum), you need to start looking for leaks downstream.

Give it a try and let us know what you find.

Thanks Craig for the excellent explanation. Tonight I found out that neither of the two lines (the red and green-striped ones) holds much if any vacuum. I'll have to investigate where the leaks are.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Well sure enough I had two leaks in the system. One next to the front passenger door where I had yanked one of the hoses from its connectors when I was replacing those rubber boots, but I didn't see it happen because it was under that black, plastic piece that protects the vacuum hoses. The other leak was more minor and it was the one that caused the weak, one-way operation of the central locking system that I had been dealing with since buying the car 3 years ago, but I never paid much attention to this matter as I was dealing with more important stuff. This leak was due to a crack in the rubber insulation of the fuel tank door actuator. I sealed the crack last night with glue and it's good to go now. Now I just have to put everything back together.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Good job, let us know how it works when you get it put back together.

Thanks Craig. It's all back together now and the system works much better. I can actually unlock all doors centrally now, and lock them again. But if the car sits for a few hours the other doors don't unlock anymore. I still have a very minor leak somewhere which could account for that, probably around one of the T's that connect to the fuel door actuator, but I'll tackle that some other time when I have nothing better to do.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Are you replacing the flexible rubber connectors as you go? (connect the semi rigid plastic to the actuators) I recently went on a mission, armed with many feet of silicon hose and replaced the roughly 2 inch long connections. Helped the system a lot. Most were probably ok, but I accidently replaced the one that was leaky. (Cheap at $1.15 per foot)

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Mercury is a boxer

1984 300d
1999 Prelude
Medici road bike
Klein touring bike
Klein mtn bike

It's a PITA to find the really small leaks. FWIW, I've had to replace the fuel door actuator itself twice in the last 4 years. I don't know if it's more prone to leakage than the other ones, or if it's just a coincidence.

I did replace some suspect rubber hoses, but not the rubber T's (or Y's). They looked ok but they may be causing the very minor leak that I still have. But like I said the system now works well enough and it's time to focus on other things. Sometimes I wish I had fewer cars, but it's tough to downsize the fleet.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Yes it is a good idea to replace all those Ts, Ys and 4 way connectors....they leak just a little as they age and they add up to a pretty fair overnight leak...almost drove me crazy trying to find a single leak...but it was mainly the 4 way connectors down beside the front seats under the carpet by the doors....kevin