Re: Can't bleed back brakes
The one on the frame is a proportioniong valve that progressively limits pressure to the rear drum brakes based on line pressure at the inlet.
The function of the round metering valve (or "hold-off" valve) under the master is to allow the rear system to see 30-40 psi (which moves the rear drum shoes out into contact with the drums) before the front discs see any pressure, to avoid disconcerting "front brakes first" on pedal application, which causes "nose-dive" at low speed.
The proportioning valve contains a spring, o-rings, seal cups, washer and a piston. Usually you can take them apart clean them and reinstall. The hold off valve contains more of the same.
If your rear flex line is old and collapsed internally you will have no rears either. A commonly overlooked source of rear brake failure.
This is a pre '70 set up on Camaros.