tuthpick

I cannot get my cruise control to work.
here is what i have done to check.
it turns on and off with switch.
it will set when the button is pushed. the light will go off when you touch the brake or when you push both buttons.
is will not set speed or any signs of wanting to.
i am thinking it is the control module under dash or the big servo unit under hood. all cables and connections on throttle are good.
i am not sure but since all the lights and switches are showing that cruise turns on and off properly i do not think the module is at fault.
the servo unit has vacuum and other than that i do not know how to check it.
i wish to buy used replacements but hope to get the best guess.
can anyone point me in the best direction.