A system is illustratively described herein that generally comprises a vacuum chamber, an ambient pressure chamber, a valve configured for unidirectional flow from the vacuum chamber to the ambient pressure chamber during a charging stroke, and a liquid filter configured to retain liquid in the pump. In some embodiments, the liquid filter is configured to retain liquid in the vacuum chamber during the charging stroke. In other embodiments, it may be configured to retain liquid in the ambient pressure chamber during an operating stroke. In more particular embodiments, a piston may be disposed between the vacuum chamber and the ambient pressure chamber, a passage may fluidly couple the vacuum chamber and the ambient pressure chamber, and the valve can be configured to control fluid flow through the passage. Additionally, a lumen may couple the vacuum chamber to a dressing to deliver negative pressure to the dressing in some illustrative embodiments.