That is a combo valve for a disk/drum car. But looking at the lines and how it’s mounted it wasn’t stock. It also looks like the brake switch may be leaking.
Hey, here's a thought. What would happen if the thickness of the rotors was under spec? Could it be difficult for the pads to put sufficient clamping force on the rotors?
It would require more fluid in the system to move the pistons far enough to grab the rotor, yes. The clamp force would be the same as long as there was enough fluid though. And of course at some point you may lose the seal on the piston and have a leak.
And the fluid thing would just show up as a low level in the master cylinder, if you topped it off it would probably work fine as the rotors only knock back the pads so far, you wouldn’t be going from fully retracted with each brake application.