The S-IVB also ongoing to thrust at a minimal amount by venting gaseous hydrogen, to help keep propellants settled of their tanks and stop gaseous cavities from forming in propellant feed lines. This venting also taken care of Protected pressures as liquid hydrogen boiled off during the gas tank. https://zanegouzf.snack-blog.com/34022254/indicators-on-apollo4-blue-plus-you-should-know