The S-IVB also continued to thrust in a low stage by venting gaseous hydrogen, to keep propellants settled inside their tanks and prevent gaseous cavities from forming in propellant feed traces. This venting also taken care of safe pressures as liquid hydrogen boiled off while in the gasoline tank. https://felixgnvch.mybuzzblog.com/13775353/not-known-facts-about-apollo-3-5-blue-plus-processor