Jeep cherokee 87 ac problems?

the vacuum supply hose is split, disconnected or plugged. it is a small black hard plastic hose that goes to a vacuum "T" that is located close to the brake system master cyl. at the firewall. it is the only hose that goes through the firewall from the engine compartment to the interior. it is also the only hard plastic vacuum hose. usually the hose is simply disconnected from the connector. it will make a light hissing noise while the engine idles.