Roger had a great thought on the trapped air in small id through the frame fittings. Navys rpv's appear ok, he simply is using two rpv's in the front as they are located after the tee splits the lines to the left & right front brakes, since his MC is below the floor the rpv's would be needed. I would think if the rpv's where backwards (check in on RPV) or not working there would be no fluid during bleeding. Like mentioned earlier the rpv's color can vary between manufacturers.
Here is Master powers definition of brake valves.
Metering
The metering or hold off valve is used in the brake system to better balance the front to rear brakes. The valve does not allow the pressure to rise at the front disc brakes until the pressure at the rear drums has risen sufficiently to overcome the brake shoe springs. At this point the valve opens to allow full pressure to build at the front brakes.
Proportioning
The proportioning valve modulates the pressure to the rear brakes. The modulation is necessary to minimize rear wheel lock up found in heavy braking and to compensate for the differences in braking conditions in front disc / rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lock up.
Residual
There are two different residual valves. A ten pound residual valve will maintain a line pressure to the rear to keep the drum brake shoes out close to the drums giving a higher firmer pedal. Without a ten pound residual pressure to the rear you will experience a spongy pedal. A two pound residual valve is required whenever the master cylinder is lower than the calipers to prevent backflow of fluid from the calipers to the master.
Combination
A combination valve incorporates metering and proportioning into one valve. These are available for disc/drum or drum/drum systems.
Adjustable Prop. Valve
The adjustable proportioning valve is used when you have a special rear condition that requires higher or lower pressure than a normal condition. You sould always use a metering valve to the front when using the adjustable