H-Bridge Instant Death
You have to understand that when you brake a motor, that mechanical energy has to go somewhere.  If you brake by shorting the terminals, it is dissipated in the winding resistance.  If you brake by applying a voltage lower than the back emf voltage, or by applying a reverse voltage, that energy is forced back into the supply.  If there is no way for the energy to return, the bus voltage (your Vm and Ground) will rise to the point where it destroys your bridge.  Typically, in this kind of application, a circuit is used to switch a big resistor across the bus when the voltage exceeds a predetermined level.