Are you working alone ? Using an easy bleed ?
If so ditch that and try the two man method same as bleeding brakes
Have u tried leaving the nipple open on the slave see if any makes its way down ?
Are there any kinks in the line or has it been squashed as you fitted the box ?
Not much help i know but worth checking
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theres a box ! im not allowed in it ! i have to think outside ! 
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