Since your going to stick with the "owners manual" story, you should have noticed it also states that it disengages with either front or rear brake being applied.

I will say it again, if your cruise does not disengage as soon as you touch the front brake lever or rear brake pedal, there is something wrong with your system.

And as you have pointed out, rolling your throttle forward will also disengage the cruise.
But that doesn't lessen the fact you have an issue if the brake switches won't disengage cruise also.

My '15 Limited works exactly the same as my '11 Street Glide did.
The slightest touch of the front brake lever, rear brake pedal, clutch lever will disengage the cruise.

Oh, and rolling the throttle forward on the "Pre-Rushmore" bikes also disengaged the cruise.

 

You probably don't have any brake lights either...

 

Mine will shut off with the brakes, just takes more pressure and is annoying to dip the front forks to disengage. After getting use to rolling the throttle i don't even try the brakes anymore unless emergency situation. Mike

 

No difference from 05, 07 and 15 disengaging and I always use cruise control regardless freeway, local or sweeping curves due to right hand wrist issues.

 

As soon as you begin to pull in on the brake lever, the brake light comes on and the cruise disengages. No "dipping the front forks" due to heavy brake action.

If yours isn't operating like that, something isn't right, and has nothing to do with it being a "Rushmore" bike.

 

My 14 SGS is exactly the same. Hard to disengage CC using the brakes. My 09 worked with just a quick tap and the cruise control would disengage.
I use the throttle roll or press the on/off cruise button.

 

I took it out for a spin to test the theory. It takes a good pull on the front break lever to turn it off. It used to do it with just a slight pull of the leaver.
Now the front fork dips. Rear same way without dip of course.
Thats why I felt it wasn't working because of the amount of pressure needed to dis engage. I don't think it is working right.

 

It is not working right.

Cruise disengagement isn't tied to brake pressure at all.
It is an electrical switch that sends a signal to disengage.

If the switch isn't working correctly (not sure if it can be adjusted) and requires you to pull the lever in more than normal before it trips the switch, then of course the brakes are going to be applied.

But the switch should trip the cruise off before you even notice any brakes being applied.
Same goes for the brake light. Just the slightest movement of the brake lever trips the switch and brakes lights come on and cruise disengages.

 

So now the million dollar question is can it be adjusted?
If anyone knows this would be real helpful.

 

i just rode 300 miles this morning, used the cruise a lot, just a lite touch of the hand brake lever disengaged the cruise, I would not be surprised if the slight movement it took to disengage cruise was not even enough to kick the brake lights on. You are certain to have a problem, the brake disengage is an essential safety feature, first thing you grab in an emergency stop is brake. Mine is a '15 Ultra.