To "hammer the brake" means to "make a sudden stop".
It is like stepping on the foot-pedal of a car abruptly and with force, just as the way we would hit something with a hammer, to stop the car immediately.
The brake is a device for stopping a vehicle;eg footbrake, handbrake;
If brake is out of control.,
then the hammer hit with the tool itself.
eg:- He was hammering the surface of brake flat;
To repair something eg the brake easily with the tool ie.the hammer.