Momentary Over-Excitation Voltage Circuit
You could use 2 relays. One a single pole relay, the other a double pole relay. 

On the single pole relay, use an output from the micro controller to pull in the coil. Then wire the 12v source through the NO contacts on the single pole relay as well as a NC set of contacts on the double pole relay then on to the brake. 

On the other pole of the double pole relay wire the 8v source through the NO contacts to the brake.

In your code put the output wired to the single pole relay to turn on when the you want the brake to come on.  Now put in a 200ms delay timer in the output for the double pole relay with will come on after the 200ms timer is complete.

This will mechanically interlock the 2 signals so they won’t ever come on a the same time while making it as quick as the relay you choose.