The S-IVB also continued to thrust in a low stage by venting gaseous hydrogen, to maintain propellants settled within their tanks and stop gaseous cavities from forming in propellant feed strains. This venting also taken care of Safe and sound pressures as liquid hydrogen boiled off in the gas https://ambiqapollo320863.ltfblog.com/26270281/not-known-facts-about-apollo-3-5-blue-plus-processor