When you bleed it, does the air start to come out as soon as the wheel cylinder is cracked open? If so, test both sides and figure out which wheel cylinder is letting the air in. I've come across this twice in my 30 years of forklift repair. Somehow, it can get in the wheel cylinder. If the air isn't in the wheel cylinder as soon as you open the bleeder then it must have a problem further up somewhere.
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