vacuum line blues 300D

I am trying to locate a vacuum leak on my 85 300D. I've isolated it to one of three hoses that go into the firewall. Two are yellow and attach to a yellow plastic piece just before the wall. the other is red and
looks a lot like the one that comes out of the firewall and goes to the shut off thingy. All three of these will nto hold a vacuum, or even create on. when i hook a tester up, I can pump and pump and not create a vacuum. one of the yellow ones will hold a pressure though. very puzzling.
My question is what are these for? I am assuming that one goes to the shut off circuit within the dash. The yellow ones I have no idea about. do I need them? can I cap them off? I am doing all this to try to get the engine to shut off when I turn off the car. So I don't think I can just cap off the red one. Any insight would be appreciated.
Have a great day, Bill
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Find and fix the vacuum leak, don't speculatively change the car's design.
You should also know that the "shut down device," the vacuum powered bellows inside the aft end on the injection pump, can fail. Suggest you first determine if the shut down is OK by seeing if pulling a vacuum on it shuts down the engine. If so, it's time to look for the leak.
Most vacuum leaks are in the door locks.
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Look closely at the yellow lines. One is all yellow, the other is yellow with a black stripe.
Yellow goes to the vacuum switch in the drivers door. The switch itself rarely goes bad. Usually it is one of the other 3 doors, the gas flap, or the trunk element that leaks.
Yellow w/black stripe goes to the vacuum reservoir that sits on top of the fuel tank. It holds vacuum in reserve so tomorrow morning when you unlock the car, all the doors unlock. If you have no leaks...... You need to pull a lot of vacuum to see if it leaks, Very rare to leak.
Red and green go to the automatic ACC. Best to test the ACC system with the key on and the center [arrow up] button pushed. With the key off, the switch over valves bleed off the vacuum in the dash.
BTW, the brown line goes to the vacuum engine shut-off.

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Thank you so much for the explanation. I now have new ammunition in my quest to be able to shut my engine off without popping the hood.
Off to battle I go... Bill
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snipped-for-privacy@gmail.com wrote:

I recently had this problem twice on my 116 300SD. The vacuum line for the shutoff was broken exactly at the point where it goes through the rubber grommet in the firewall. While probing around, it broke off in my hand. Six months later, same thing with the line for the door locks. So, I'd take a good look there. Now as to why it happens, that is a real mystery. The lines didn't make a sharp bend or anything. I'm guessing it might be due to vibration or temp change.
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