I di have a faulty actuator on the left end of the heater ducting. Above the gas pedal. Removing the spring in it cured the problem. But I also agree with the other guys. It's more apt to be a vacuum leak under the hood. Those hoses between the master cylinder and the ABS pump like to rot and crack or come off. Very common issue. The small ones are 5/32" if you need to buy some new hose.
