Here's a question: isn't a vacuum the absence of air? Why a cannister?

Here's my question: Why does a car need a vacuum cannister? Mine is broken, it's in a crappy location to access and I want to replace mine.
So, I am planning on buying a vacuum cannister and mounting it in another location. But why do I need it?
Right now I have the tube that goes to the cannister shut off. My vents work better, but I don't see the difference between tying off the end of the tube, or connecting it to a cannister.
Vacuum = no air. Tying off = no air. Why do I need to contain 'no air'? If the cannister has a one-way valve to let air escape, why not just a check valve?
So, there goes. What purpose does a cannister serve versus tying off the tube?
Jamie
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Jamie wrote:

Since you mention "vents", I assume you are talking about the vacuum reservoir in the lower dash. You are half right: this isn't needed to operate the ventilation controls *with the engine running*. What it does do is provide enough vacuum to operate the controls for a brief time when the engine is shut off, or when engine vacuum is especially low.
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Sorry if I was vague. The cannister I am referring to on my 1987 740 GLE non-turbo is buried under the front bumper of the car. I could barely even see the cannister by crawling under the car, so there was no way for me to assess the damage without apparantly unbolting the front bumper.
There is a vacuum line that comes through the firewall to the engine where it is t-connected to a hose going through the intake manifold and another to the vacuum cannister.
There is a check-valve on the hose from the intake manifold to the t-connector. Well, I disconnected the tube going to the cannister and I blew into it and sucked. There was free flowing air both ways, so I figured something at the cannister broke. So, I removed that tube from the t-connector and sealed off the end of the t-connector. With the tube connected to the broken cannister I had little air coming out the A/C vents. When I sealed it off, a LOT of air came out the A/C vents.
I want to buy some generic cannister and mount this somewhere under the hood where I can easily access it. My only real question was do I actually need one.If so, how does a cannister store a "vacuum" when a vacuum is nothing?
mjc<DELETETHIS>13 wrote:

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Wouldn't the canister be acting as a sort of ballast. Under negative pressure the canister would be to the left side of zero (vacuum) as opposed to zero (nothing)
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Now that's interesting. Are you saying that by sealing off the connector, I get zero pressure versus some cannister that actually would cause negative pressure, or sucking, or in other words 'create' a vacuum?
I can sort of see that. My confusion was I read that the cannister 'stores a vacuum' and this didn't compute. If the cannister is somehow enhancing a vacuum by maybe providing a volume of area that would somehow serve as a tool to create that 'left of zero' you speak of, I see where you are going.
Thanks!
KLB wrote:

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Jamie wrote:

Whether it's vacuum or air pressure is irrelevant, it may be confusing if you're making it too complicated but there's still pressure, it just is in the opposite direction. You can "store" vacuum just like you can store pressurized air, you're limited however to around -14 PSI. If you had a big evacuated can of "nothing" you could connect it to something else and draw a partial vacuum in that, just as you could connect a pressurized tank to something and pressurize it.
If you tie off the vacuum cannister your vents will close when you stomp on the gas and the engine vacuum drops.
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Thanks, the picture is becoming clearer.
Would it be fair to say that the main difference between tying off the tube, versus having a cannister, is that without the cannister you are limited only to the amount of vacuum stored in the tube. With the cannister, you have a greater volume of vacuum.
When you stomp the gas, the vacuum in the tube is quickly depleted - versus having more vacuum in reserve in a cannister?
James Sweet wrote:

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Jamie wrote:

Yes exactly, just as if you were trying to store pressurized air to actuate pneumatic solenoids, which is exactly how the vents are operated except vacuum is readily available from the engine so they use that.
It really isn't hard to remove the bumper to get at the cannister, just a few bolts.
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Ok, I'll give another looksee. I would like to put my new pneumatic drill to use. It's a shame to have all that torque and power just sit there. ;-)
JB
James Sweet wrote:

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I'm guessing you have an older model. Our '85 765T has a vacuum canister, and I repaired a crack in it with JB Weld years ago... no trouble since then.
The vacuum canister holds the vacuum motors that control the ventilation system in the selected position when the intake manifold isn't producing much vacuum - hard acceleration or hill climbs. If you have a turbo model there is also a vacuum pump to maintain the vacuum for long hill climbs, and that needs the canister so it isn't cycling on and off rapidly.
Mike
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