Ok.
We just had a member on our Aussie forum with this exact same problem. His gremlin turned out to be mal-adjusted tone ring pick-ups, on the rear wheels. The gap was TOO BIG.
This tricks the braking system into thinking the rears are locking and reduces fluid pressure. The HCU is simply doing its job. But with incorrect information.

My advice is check the gap on the tone rings/pick ups on the rear wheels, adjust, and go from there.

HTH...

cheers