First I would remove the booster and throughly inspect it. As long as there is no damage to the rubber inside, it should be OK after a complete cleaning. The brake fluid can only come from the master cylinder, which obviously needs to be rebuilt or replaced. A lot of times they will leak fluid either on to or inside the booster after bleeding the system. To prevent this put a piece of wood, like a 2x4, on the floor so that the pedal doesn´t go all the way to the floor during brake bleeding.

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