Cruise control not holding speed

It does sound correct. The fact that the green light comes on means from the switch to servo are operating. The fact that it does not engage is either the servo or any input that is telling the servo to disengage. This could be the dismiss switch is remaining engaged or the brake switch is sticking just enough to disable the system.

Do you have a scan tool that can monitor the input of the brake sensor and see if the PCM is reading any voltage with the brakes not depressed?

If not, the servo is the most likely cause. I attached the process for this and with nothing else to go on, I would just go ahead and replace that and retest. If that doesn't fix it then we will need to get access to a scan tool or we will just be guessing. Thanks