Have you definitely got it plumbed in right?

Normally on hydraulic handbrakes you plumb the master cylinder for the handbrake in backwards. You connect your rear circuit master cylinder 'out' from the pedal box to the handbrake master cylinder 'out'. The handbrake master cylinder 'in' then goes to the rear brakes.

This allows the fluid to flow through the handbrake for normal brake operation, but will still pressurise the rear brakes when the handbrake is operated.

I've never had to do anything but bleed as normal on the rear brakes.