If a system looses pressure, the valve shuttles and stops the fluid to the failing system top keep the operable sytem functioning, as a safety device.
Check to see that pressure is being sent to each system, front and rear alike. pressures will be different of course, but as long as you get action that would be an indicator the valve is not shuttled to one end closing down one system.
A RED BRAKE warning light on the dash that is illuminated( after bulb check) is a good indicator that the valve has shuttled and not operating both systems.
Fact is stranger than Fiction