Piloted Industrial Back Pressure Regulator, Sizes: 1-1/4″ – 24″ In many liquid piping systems, it is vital that line pressure is maintained within relatively narrow limits. Installed in the main flow line, the standard Mark 5108-2 acts as a back pressure or pressure sustaining valve. In this configuration, the back pressure sustaining valve maintains a constant upstream pressure regardless of fluctuating downstream demand. When used in a bypass line, the same model will function as a relief valve.