Hello, I'm Chris, a current Ford Lincoln Mercury employee.
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All vacuum supply comes from the intake manifold. So this narrows it down a little. A vacuum hose not connected will produce a hissing a sucking noise when idleing and when sprayed with ether or carb cleaner or brake cleaner it will increase the idle. I believe the port it connects to is on the rear corner on the passenger side of the intake,.
Check the vacuum line to the EVAP service port on the firewall. It will have a green cap.
the red vacuum line goes to the vacuum tank and then goes off a black and white check valve to a thin black hose ,, this thin back hose is the main vacuum line to the a/c inside the van,, then thin vacuum line goes into the fire wall and into the van for the a/c ---its hard to see this unless you have a flash light and the engine is cool ---i have seen holes in the red hose and the black hose and in the vacuum take on this unit,, also the chk valve can be bad --- check the red and black hoses for cracks or holes ,, if thats ok ,,, remove the check valve and blow and suck on it ,,it should only let air go one way ---if thats ok ,,,then cap off the line going to the vacuum tank and see if that makes a difference --tell me what you get