mazda MPV brakes

My kids Mazda MPV 4WD with rear wheel "ABS" has a low brake pedal. I
took the master cyl off and stripped it down, washed it out, rubbers
looked good, cylinder clean and smooth, re-assembled and began to
bleed the brake system. I got no improvement in the brake pedal -
still way too low.

Rear drums removed, shoes & hydraulics inspected, brakes adjusted
manually to "just" rubbing. Front pads will need replacement
sometime this summer, but still serviceable. No leaks in any of the
cylinders or calipers. The right rear has no bleeder. It has a
brake line going in and one going out to the left side. The rear
brakes are in series with the right side feeding the left - and then a
bleeder. Bleeding the rear brakes at the left, bleeds them both.

There is a bleeder on the "ABS" module that feeds both rear brake
cyls.

I must be missing something really obvious, and I can't for the world
of me think what. Before I go and strip everything down to nothing,
I need someone with the "obvious" answer.

Thanks

Pete