[robnolimit]

Good question, and by all means, question away. Here's what a RCV (Residual Check Valve) does. It's kind of a one way valve, all of the fluid flow and pressure can go one way, "To the caliper or wheel cyland when the delivery pressure is gone, all of it can return (to the master) until the pressure drops to a certain level (2lb or 10lb). So, why do you want this?
There are forces trying to push the fluid back to the master, and you want this to happen, to a point. You lift you foot, line pressure drops, the pads or shoes relax, that's the pressure force, drums have big return springs that would continue to push fluid back until the wheel cyl was completely in, - some High performance calipers have return springs as wee. Then, the heat differential comes in, hot fluid pushes towards the cooler fluid, and lastly, any run out in the rotor or drum, bearing slop, axle or spindle flex, caliper mount flex and other forces are all trying to push the fluid away from the caliper or wheel cyl. But you don't want that to happen. You want the pads or shoes to stay as close to the brake surface as they can, without (or with very little) contact. That way, next time you hit the pedal, the brakes are nice and consistent and high. the same every time. If you hit the brakes once, and then quickly release and hit again, and the pedal is higher, you have too much fluid returning.
So, even up on the firewall, the drum style return springs can easily push fluid up against gravity, and the heat and run out of the rotors can do the same. The residual valves will let the fluid come back, yet hold enough in the lines to overcome these additional pressures. Hope that makes sense.