That fluid has got to be going somewhere, *IF* it's truely
leaking down. It's possible, that it could be the bleeding technique.
Presuming the rear shoes are adjusted up properly...............
Pay attention to the order that you bleed the lines, and run a couple bottles through while you are bleeding them again.
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To find a hidden leak:
Parking brake down, (it's really only for parking),
Pull the wheels, spray brake clean on every fitting at the wheels
and master cylinder, spray down the calipers/wheel cylinders,
and run a bottle or two of fluid through while you are bleeding
them again.
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What made you start the fluid bleeding process in
the first place/what changed? New pads? New shoes?

Let us know what you found.-Bob