It's just basically there to allow a pressure sensor to be hooked up inline with both front and rear brakes, and warn you of low pressure in the system. It also acts as a divider for the front calipers.

It does more then that. There's a shuttle valve inside that will close off either the front or rear braking system if there's a hydraulic pressure variation between 187 and 250 psi. Its designed to keep from pumping all the brake fluid out of the system and give some stopping ability if a big leak occurs.

It can also really screw up a brake bleeding job if you don't gravity bleed the complete system before pumping the brake pedal.

Hmm...ok, so this confirms what I mentioned above, about another thread I'd read illustrating this problem. I need to look into that, I think. I haven't yet opened the PDF service manual, so if it's in there just say so and I'll check it out after work, but is there a way to reset that valve if it's been tripped?

It does more then that. There's a shuttle valve inside that will close off either the front or rear braking system if there's a hydraulic pressure variation between 187 and 250 psi. Its designed to keep from pumping all the brake fluid out of the system and give some stopping ability if a big leak occurs.

It can also really screw up a brake bleeding job if you don't gravity bleed the complete system before pumping the brake pedal.