Can anyone tell me when cruise control is set, should it slow the vehicle by applying the brakes when going downhill to maintain the set speed.

No

 

No, it just shuts of the fuel i believe some autos it will apply  the brakes but not on fords.

My 2013 Escape will actually down shift as speed increases on down hill runs.  

On manuals, the CC only operates on throttle, not on brakes, so if the hill is too steep, or the revs too low, it won't keep you to the speed set...

My focus is automatic and no it does not apply the brakes to maintain the cruise control speed.

 

If you're worried about getting flashed by a speed camera, better off putting it on the limiter and the bell will go off when you go above speed. Control the braking with your foot in those cases.

 

I've found CC only really useful on long motorways myself on pretty much any car I have owned with CC. 

It's annoying that it doesnt auto brake unless you have specced the additional active cruise.  My 2008 C class did not have radar but it would auto brake downhill.

Given that the collision assist can auto brake it could have easiuly been set as standard.