Cruise control with a mind of it's own

Hi Everyone

I just bought a 230E w123 1983 model, what a car!
I was wondering if someone could tell me how the speed cruise control works. It would seem that once set it applies brakes when going down hill to maintain the set speed. Is this how it works? From time to time when using the speed cruise control and the car is on a level road the cruise maintains the correct speed but you feel the same sensation of the car braking. Why would this be?
Today I even had it going up a hill!!!
Help!

Norman