"break" vs. "brake"
As Vidamuzo pointed out in a comment, you probably mean "to brake" (take an action to decelerate a vehicle), not "to break" (destroy something, or rupture it into two or more pieces).
Similarly, to do that, you'd use the vehicle's "brake(s)", not a "break" (interruption of an activity or of work, a pause).
The Esperanto words for "to brake" and for "brake" (the noun) are "bremsi" and "bremso". ("paŭzo" would be the "interruption" meaning of "break" (noun).)
what kind of "hard"?
"hard" in English can mean many things, among them "the opposite of soft" ("malmola"), but also "difficult" ("malfacila"), or strongly / with much force ("forte"). Here, you're looking for the latter, thus "forte" should probably be the adverb used, maybe with a "tre".
putting it together
to brake hard
(tre) forte bremsi
or (at your preference)
bremsi (tre) forte
Sofia brakes hard.
Sofia (tre) forte bremsas.
Sofia braked hard. / Sofia did brake hard.
Sofia (tre) forte bremsis.
Sofia will brake hard. / Sofia is going to brake hard.
Sofia (tre) forte bremsos.