I hesitate to tell you what you most likely already know, especially since I have (thank goodness) never been under the 124 dash. It is most likely a leaky actuater diaphragm, but check all the exposed vacuum lines and connectors closely. There is a mess of them up under the cowl behind and above the master cylinder. Failing finding the problem there, it's not too much trouble to get the under dash panel down, so maybe you could get lucky and find a rotted or disconnected vac line there.
Best of luck,