Lem,
Again you are correct that the answer lies in the shuttle valves. These valves are designed such that the priority system ( in this case B) has a larger surface area on its pressure face than the alternate system. (think of the valve as a stepped shaft inside a tube with inlets at each end and an outlet in the middle). In this way when both systems are at the same pressure then the side of the valve with the larger pressure face has greater overall pressure acting on it than the smaller alternate pressure face. The valve then slides over to allow B system fluid to operate the brakes. When B system pressure falls to below 1500 psi (the ratio of the faces surface area is approx 2:1) the overall pressure on the A system side now overcomes the B system side and the valve slides over to allow A system pressure to operate the brakes.
Does that help??