Back in the 70's they sold a tool that would screw into the hole where the brake failure warning switch screws into the proportioning valve... It would lock the shuttle valve in the centered position allowing your to bleed the brakes... usually it isn't needed but that may be what you have going on... Basically there is a groove cut in the center of the shuttle valve so switch is open.. If pressuse is lost to either side of the shuttle valve the spool will shift... blocking the fluid path to stop the fluid from leaking out of what ever failure exsists on that side of the system... Also when the shuttle valve shifts it grounds the switch which would turn on the brake failure warning light.... You might try removing the switch, use a scribe to center the valve then try bleeding the brakes while locking the spool in the centered position...