No fluid at all flows through the bleeders? Could be the inter-circuit seal that keeps the front and rear sytems seperate, if that is bad you will not get any pressure to the bleeder but will work normally when the bleeders are shut. The seal is in the master cyl and is the middle seal with springs on either side. I would check out the casting too, an open pore in the bore would bleed off pressure too.
We kind of need a better explanation of the problem to help you diagnose further.
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