drum brakes need that 10 psi to keep the shoes out near the drum----disc brakes only need 2 psi to keep the pads next to the rotor----just having the master cylinder mounted on the firewall gives a column pressure near the 2 psi number---ditch the residual wherever it is....you could run a new line from the master cylinder to the rear brakes or make a short bypass line at the residual.....