Recently, the continuously pumping oils through porous hydrophobic/oleophilic materials has shown to be an attractive technology to separate oil from oil/water mixture (i.e., bulk oil spill cleanup and micro-oil droplet removal). This new technique applies external vacuum pressure through the porous materials to continuously suck and separate oils in situ from oil/water mixture with high speed and efficiency. However, one issue associated with this pumping-through-process is that water might be sucked into the porous materials due to the external force applied even though the porous materials are water-repelling. By looking into the intrinsic nature of the oil sorption process into the porous materials, we found that the sorption process is merely a capillary phenomenon. A simple force analysis on the meniscus of oil-water interface, which locates at the entrance of micropores of the porous materials, can help us explore a safe working vacuum pressure range at which only oil would be sucked into the porous materials while water is repelled. The upper range for that safety working vacuum pressure is the percolation threshold vacuum pressure. Beyond that threshold pressure, water starts to flow into the porous materials as the external vacuum pressure is higher than the capillary pressure that repels water. Then, we used a self-fabricated open-cell hydrophobic/oleophilic foam to experimentally measure the percolation threshold pressure. The theoretical prediction as well as the experimental validation about the percolation threshold pressure will provide direct guidance for process design and operation control.