[Gaz64]

Personally, I think you are playing with fire.
For starters, electric park brake is on or off, no in-between as determined by driver hand modulation, like with a normal mechanical handbrake.
And in factory fitted cars, when the park brake switch is operated in emergency above a certain road speed, say 20mph, the ABS module applies the normal braking system on ALL 4 wheels to help stop the car.
I doubt very much at all whether you will get a working system for emergency use if necessary. Look at any rally or drift car that uses the rear brake only to set the car up for cornering.
It is NOT a normal handbrake, and CERTAINLY not an ELECTRIC handbrake.
Gary