67 Chevelle w/ front disks. Pedal's been a little spongy last couple of drives and went to the floor today and "brake" light on the instrument panel came on. Checked the reservoir - one side filled to the brim, other side bone dry. No signs of leaking anywhere. Gonna bleed the brakes tomorrow, but anyone have any thoughts on why this happened.

You have a leak in the system. Pull ALL of your wheels and check for a leak, either in the form of a "blown" wheel cylinder or bad caliper. If there is no brake fluid leaking from the cylinders or front calipers, check ALL of your brake lines starting from the rubber lines at each front wheel(and the steel lines to each side of the car). Proceed all the way to the rear of the car checking the steel lines for rusted out sections. (my son's 86 Olds front to rear steel line rusted out where it was held to the chassis by a mounting clip). Above the rear "pumpkin", check the rubber hose which connects to the rear brake lines which travel along the rear axle tubes. A word of advice here: the two lines which travel along the rear axle tubes rust out quite frequently....check them carefully. The rubber connecting line also fails after many years of service. Use a good light source to help you check the brake lines. I've had rear brake lines fail, front rubber hoses fail, brake cylinders "blow", and the rear rubber line develop a crack. During my 614,000+ miles journey in my Chevelle, every brake problem known to man has reared its ugly head! Check the entire system CAREFULLY. Your life, and someone elses, might depend on your brakes. Good luck.

Thanks! I will look in to it further based on your recommendations.