polymer filtration apparatus comprising a body having a pair of upstream passages each terminating at a slide channel and a pair of downstream passages respectively opposing the upstream passages , and a slide member in the channel having apertures for a pair of replaceable filters each normally communicating between an upstream passage and a downstream passage . the extents of the channel surfaces between the pairs of passages and between each passage and the exterior of the body , and the extents of the filter apertures on the slide member are related so that one filter remains active when the slide member is in position for replacing the other filter . after a filter has been replaced the new filter may be prefiled without pressure loss in the polymer or excessive leakage thereof .