My rear brake is too bad and needs improving

Both of those points have been covered already. I would hope that those would have been checked by now.

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otakar wrote:

Both of those points have been covered already. I would hope that those would have been checked by now.

here is the OP's response:

The pads and fluid are as new as the disk

Then later:

The disk, the pads and the fluid was changed at the same time.

What does "as new" mean? Used in good condition? Are they original? Come off another bike? When the disk was changed, was it surfaced or just thrown in with new pads? I'm still wondering.

Schorly wrote:

The disk, the pads and the fluid was changed at the same time. I didn’t change the hoses and may have not flushed the system through enough to get rid of all the debris. So I will do this next and see if the braking improves. If not I might have to live with the inefficiency of the rear brake as it has been mentioned by other comments.

I don't know who's telling you the rear brake is "inefficient". Mine will lock the rear whell no problem with a good tap of the toe. If yours won't, something needs to be fixed. The old KZ brakes may not have state of the art heat dissipation of dual disc modern bikes, but they will certainly grab the wheel and lock it if you stand on them.