Check all vacuum hoses for cracks leading to leaks, or hoses having fallen off. However, I'm quite sure you have a vacuum motor out. These are the bulbous looking canisters that are actuated by vacuum, and if the rubber badder (bellows) starts leaking due to age cracking, you'll have the symptoms you described. These motors have really small hoses attached to them, fed by a hose from the engine bay to your on/off switch and then to those motors. They can be checked with a Mighty-Vac, or just suck on them really hard. You should see good movement by the small arm that sticks out from the motor. I've tested them up to 25 pounds, both vacuum and applied pressure. There should be 3, maybe 4 of those motors, all under the dash and attached to the heater core housing.