A seal section of a submersible well pump assembly has an expandable and contractible bag surrounded by a pressure equalizing chamber. A motor lubricant communication path communicates motor lubricant the bag interior. A well fluid port admits well fluid into the pressure equalizing chamber. A gas vent passage leads from the pressure equalizing chamber to the exterior of the enclosure. At least one membrane in the gas vent passage allows gas contained in the well fluid in the pressure equalizing chamber to vent. The membrane blocks liquid from flowing through the gas vent passage into and out of the pressure equalizing chamber.