Wasn’t really commenting on OP just wanted to keep some from a potential mistake. Brakes don’t really have back pressure so to speak. Just pressure then normal pressure (compression,decompression). When the brakes are applied the mechanism pushes on the fluid which has an effect like pushing on a plugged syringe…only the piston has give and that pressure forces the piston out to relieve excessive pressure and blowing a line or master body. Let off the pedal or lever and that pressure relaxes. Little to no back movement. Add ABS and you add a pump body that keeps a preset pressure in the system and monitors it. When it detects an excessive amount in a short time it springs into action by modulating the pressure. The only time more fluid is used is to fill the extra room left when the pads wear thinner
Another thing to add is there is no suction or back pressure to get the pads off the rotor. That job is left to the piston cup seal. It flexes with application then draws itself back to its original state from the beginning.