Additive manufacturing (AM) has enabled production of novel liquid hydrogen (LH2) fuel tank configurations using polymer matrix composites (PMC) reinforced with carbon fiber and glass beads for aerospace applications. However, in experimental field trials hydrogen leakage into insulation layers of prototype tanks reduced LH2 storage duration up to 65%. Four modes of hydrogen leakage through the tank constituent material were experimentally investigated: 1) permeation through the AM PMC, 2) flow through cracks induced by thermal stress, 3) flow through unsealed epoxy joints, and 4) bulk flow through catastrophic failure. Measurements were completed with AM PMC tank specimens at both standard and cryogenic temperatures through a combination of mass spectroscopy, optical microscopy, and burst testing. The sample measurements were compared with the ability of full-scale tanks to retain LH2 with no leakage. These results indicate that unsealed or broken joints within the tank were the cause of poor insulation performance, and no hydrogen seepage was detected via other transport mechanisms through the AM PMC.