The effectiveness of biomimetic dry adhesives at different ambient pressures was investigated. Biomimetic dry adhesives have great potential for space applications but there have been few studies on how these adhesives perform in low-pressure environments. The best performing geometry for dry adhesives with respect to normal adhesion has previously been determined to be mushroom shaped fibers, but the pressure sensitivity of these designs has been unclear - with some groups reporting pressure dependent adhesion, and others claiming no effect. We have compared the microscale adhesion of mushroom shaped polymer fibers at different ambient pressures and have determined that suction cup effects are negligible for fibers with caps less than 16.4 mu m in diameter. Further investigation in this work showed that a simple suction cup model provided estimates for the geometry requirements for a non-negligible suction cup effect and determined that the minimum cap radius for the 10 mu m pillar diameter used in this study should be over 26 mu m - past the dimension that can be successfully fabricated using this technology.