You will want to wash your bladder before your first use: Rinse out your bladder with lemon juice or mild soap and warm water to remove any residues—then let your bladder dry out completely before reassembling it. You may need to prop the bladder open with paper towels to allow it to dry completely.
Once it is dry, reassemble it. Be sure to tighten the hose at the base of the bladder—the O-ring gasket needs to sit down into the receiver so it can seal properly—this will prevent leaking. To see this demonstrated refer to this video (minute 1:25).
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