First hold the brake pedal up with your toes to see if it will engage. There should only be one circuit for both the brake lights and cruise turn-off signal but if the switch is out of adjustment it could be turning on momentarily from bumps in the road. Check the brake light switch for the presence of a vacuum hose. That hose is a backup cut-off signal in case the brake light switch fails. Be sure it isn't leaking. Also check the vacuum hose to the servo to be sure it isn't leaking. They usually tap of from the power brake booster. To find the servo, follow the second throttle cable to it. Pull on that throttle cable too to see if it is broken. If it is, you might be able to find one in a salvage yard. We installed a lot of aftermarket cruise controls when I worked at a Sears Auto Center in the 1980s. GM owners had a common problem with defective servos and they were no longer available from GM by the time those cars were three to four years old. Any that you found in the salvage yards were usually defective too.