I've got a diagram that shows a yellow with gray (maybe green, that part did not print, so I filled in) stripe going to the reservoir only. If so, I would think you would have to evacuate all air that's in there before you can tell if there is a leak some where. However, check the reservoir's connection, to fix, use non hardening silicone, making sure to not clog.

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83 SD

84 CD

Actually, now that you mention it, it is grey.

Okay, I think that at the firewall, I have two yellow lines – one with a grey stripe, and a solid yellow.
The grey, you say, goes to the reservoir? Okay.
That means the second solid line goes to the driver’s door, where it splits to red and green (lock and unlock).

Sound okay so far?

I would think I should be able to build a vacuum in the reservoir unless it was broken. But, I shouldn't be able to unlock and lock back my doors if the reservoir was bad, right?

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1982 300CD Petrol/Black Leather

I pulled the line at the reservior in the trunk and hooked the Mityvac to it. It goes up to 15-20", but slowly loses about 1" every 10 seconds or so.

Slow leak SOMEWHERE.

I am thinking this might be associated with the delayed "flop" of the vent doors.

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1982 300CD Petrol/Black Leather

The diagram shows that striped yellow line "starts" at the nipple closest to booster, to a connector, shows lock system only, to a check valve, to another connector, one of which is the yellow stripe, the other, goes to another check valve that goes to ignition switch. Then straight to resevoir. When you checked the resevoir line, the other end was connected? That would be to a check valve that goes to the ig switch. I would think it would hold vac. The solid yellow line does go to the door locks. Drivers side first, then branches off in a series of connectors, to eventually the fuel door lock. If you have the lines, check the resevoir itself for leaks, will have to pump like a....well a long time.

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83 SD

84 CD