That is what it is supposed to do but...
What are the RPM you are seeing?
- 3000-4000RPM, perfect.
- If it gets above 5000RPM, give it some brakes to keep the RPM below 5000.
When you use the brakes going down hill, try not to build up too much speed. Once you are going fast, it's much harder to get it slowed back down. When it gets too high, apply the brakes firmly so it slows down 15-20mph and the RPM gets down to a more reasonable level. Then let it creep back up.
Do not just ride the brakes lightly. This will heat up the brakes until they are literally red hot. At that point, they will lose a lot of their stopping ability.
By using short hard braking, they have time to cool off between uses.