SOURCE: 2001 Chevy 1/2 express van.A vacuum line under
The vacuum line from the engine's intake manifold goes to a vacuum reservoir ball under the hood; from there a vacuum line leaves this reservoir, passes through the firewall and connects to the mode control switch.
The mode switch directs vacuum to actuators that position the doors to the desired vents. When there is no vacuum the doors are pulled by springs to a rest position; defaulting back to defrost.
The vacuum reservoir is used to store vacuum when the engine is under load and not producing large amounts of vacuum. The inlet vacuum line may have a check valve; stopping vacuum from going back to the engine. Make sure that this valve (if there) will only allow vacuum to pass through it in one direction.
The lines that control the AC and heater all pass through the firewall and some are up around where the wipers are if there are no lines coming out of the firewall except the one by the brake booster then simply plug the line coming off the T with a screw and see if that restores your vacuum to the controls. You may just be leaking so much your controls arent working. These lines that run around the manifold tend to disenegrate over time and with out seeing the issue I have to guess that the one unconnected line is either broken off and should connect to something withing its length or the hose itself could be cracked in half which leaves the other half hanging out there somewhere.
You might need to remove the center console & the engine cover in order to access the main vaccum line that is right in front of the PCV valve on the driver's side of the engine. Make sure you have a really long flat head screw driver to remove the two slotted bolts on the engine cover on the firewall. Also, since the placement of the vacuum hose is definitely a design flaw as it cannot stand up to the heat of the engine for more than a few years without degrading the line, make sure you invest in a heat shield wrap or another larger vacuum hose to engulf the replacement hose & to act as a heat shield. I recommend replacing all of your AC vacuum hoses at the same time
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