Due to space considerations it appears that the brake reservoir(s) are remoted in RHD cars. In LHD applications the reservoir(s) are mounted directly to the master cylinder (aka, servo). The rear half of the reservoir(s) supplies the rear brake circuit, the front half supplies the front circuit. [Which half supplies the clutch circuit? The front?]

Look to the connection from reservoir to master cylinder as one possible leak point, also examine all the brake hydraulic lines that connect the master cylinder to the rear brakes for leaks, and the rear calipers.