Cant bleed brakes with this hooked up
 

Years ago I bought a 69 Grand Prix & installed the disc brakes in my 68 Le Mans convertible. It has sat for a few years so I replaced the front calipers & new master cylinder. Now I can't get the brakes to bleed. I blew out all the lines & found that the "brake hold off valve" (at least that's what a seller on E-bay calls it) wouldn't allow bleeding. I disconnected it & installed a temporary line in it's place but now after bleeding the brakes they still go to the floor.

The seller on E-bay stated that this "hold off valve" needs to be rebuilt. Any suggestions on what to do with it?