Ditto. ditto all above! How about the steel brake lines? Are we talking new? If not, maybe use air to clear the lines, blast the internal gunk into a pail disconnect hoses and clear out one at a time. Use inverted flare plugs so no crud is blasted other than the pail. Inspect lines and possible pressure test. If all checks out, then flush bleed as recommended by others.
Otherwise: make new good lines of known quality. The residual pressure checks are not intended to block two way flow only to retard somewhat the flow back to the mc.
