Cruise control help needed

I have a 79 heritage edition with cruise control. I cant seem to make the cruise work like it should. The buttons all work like they are supposed to. It catches immediately when you press the button, but it just keeps accelerating without end. I have fixed all the vaccum leaks that I can find, and I have tried leaving the chain loose and tightening it up, still the same. Any ideas anyone? I don't know anything about these older cruise systems, and I want to take some long trips in this car!

Sorry not to reply sooner. Have you done any checks on the amplifier, servo assembly, VSS (vehicle speed sensor, mounted in-line with the speedo cable), even the on/off-coast/cruise buttons need to be checked with ohm meter. I went through all these tests when I decided to re-install a factory style CC in my Ranchero, but the factory button location was an issue(aftermarket steering wheel), so I installed an aftermarket unit.