'C3 78 underdash hissing

[Jay #36173]

When the car is running I get a vacuum sounding hiss from under the dash on mt 2300 mile '78. I'm assuming it has something to do with the HVAC system of diaphrams an hoses. Any ideas how to try to start diagnosing the mile of rubber plumbing under the dash? The noise occurs regardless of HVAC switch position. Also, all positions work correctly (i.e. a/c, vent, heater and defrost) although the a/c either needs just an evac. and recharge or a compressor rebuild (based on past experience, suspect the latter will be necessary).

Thanks in advance for your suggestions.

[Jack H.]

Re: 'C3 78 underdash hissing

There are vac lines in the cockpit to service the headlights (on/off switch AND bucket over-ride switch), the heater-A/C controls (at the control panel adjacent to the shifter), and the make-up air servos (doors in the RH cowl and kick panel selecting inside or outside air--recirculate function).

How to diagnose? You roll up your sleeves and go find the hoses that are either disconnected or leaking. Hissing noises can be anywhere from the far LH side of the instrument cluster all the way across to the RH kick panel. Usually, this happens because someone before you did service and forgot to re-connect one or more vac lines....