Your '79 Cordoba is "not" too old to notify you of low brake fluid level or a failing system, when the master cylinder failed on my '70 Challenger R/T, the parking brake (emergency brake) light illuminated, fortunately I had already felt the impending failure in the pedal and was downshifting (four speed) and giving lots of room at traffic signals and such on my way home.
I would say you have a braking system that is failing and as you sit at a stop for a long period, the pedal is fading under your foot, fading enough to illuminate the light. Check your fluid level immediately and if full you have other problem(s) in the system.
Don't hesitate, the brakes may not be there when you most desperately need them!