Master brake cylinder

The brake master cylinder converts the force applied to the brake pedal into hydraulic pressure for the service brake. This component is mainly found on hydraulically braked vans, such as the MAN TGE and Mercedes-Benz Sprinter, Vito, Viano, V-Class, Citan, TN and Vario. On heavy trucks, buses and coaches with pneumatic service brakes, including the MAN TGL, TGM, TGS, TGX and Mercedes Atego, Actros, Arocs, Citaro and Tourismo, a foot brake valve performs the equivalent metering function using compressed air. Therefore, never select a brake master cylinder based solely on the model name or year of manufacture: identification by chassis number is always essential.

A brake master cylinder generally consists of a cylinder body, brake fluid reservoir, primary and secondary pistons, sealing cups, compensation ports and connections for separate brake circuits. The tandem master cylinder version operates two hydraulically separated circuits. In the event of a leak or pressure loss in one circuit, the second circuit must retain limited residual braking performance. The bore diameter, piston stroke, position of the connection ports, mounting fitment to the brake booster and type of reservoir level sensor determine whether a cylinder is technically interchangeable.

On vehicles with ABS (Anti-lock Braking System) and ESP (Electronic Stability Program), the brake master cylinder works together with the hydraulic ABS/ESP control unit. A different bore size affects the displaced fluid volume and therefore pedal travel, pressure build-up and ECU (Electronic Control Unit) intervention. On the MAN TGE and Mercedes Sprinter, variants may also differ according to wheelbase, permitted gross vehicle weight, front- or rear-wheel drive and brake disc diameter. In addition to the chassis number, always check the number of hydraulic connections, thread sizes, distance between mounting bolts and brake booster version.

A brake pedal that gradually sinks under constant pedal pressure often indicates internal bypass past worn primary or secondary sealing cups. External leakage is visible as brake fluid around the rear of the cylinder body, the brake booster, the line connections or the reservoir. A hard pedal with limited braking performance, on the other hand, is more often caused by a faulty vacuum brake booster, insufficient vacuum supply or a blockage in a hydraulic brake line than by the brake master cylinder itself.

First check the brake fluid level and inspect the fluid for dark discolouration, moisture contamination or rubber particles. Then measure pedal travel and assess whether it changes when brake pressure is held for an extended period. Following the prescribed manufacturer procedure, temporarily isolate the outgoing hydraulic circuits using suitable adapters. If the pedal continues to sink, internal leakage in the brake master cylinder is likely. If pedal pressure remains stable, inspect the brake calipers, wheel cylinders, flexible brake hoses, ABS hydraulics and line connections for volume loss or leakage.

Also read fault codes and live data using the diagnostic tester. Messages relating to a brake fluid level sensor, pressure sensor or plausibility fault do not automatically mean that the cylinder is defective. First check connectors, broken wiring, corrosion in connector pins and the reference values of the relevant sensor.

Replace a brake master cylinder in the event of internal leakage, corrosion in the cylinder bore, damaged connection threads or leakage into the brake booster. Reusing old sealing cups or fitting an overhaul kit without checking bore tolerance and surface quality is not recommended. Only use the brake fluid specification prescribed by the vehicle manufacturer; mixing unsuitable fluids may cause seals to swell and adversely affect viscosity at low temperatures.

Fill the reservoir cleanly, prevent the hydraulic system from running empty and bleed it in the prescribed sequence. On vehicles with ABS/ESP, a diagnostic bleeding procedure may be required to completely remove air from the pump module and control channels. After installation, check pedal height, pressure retention, leakage and fault memory. Correct chassis number selection and technical support with identification help minimise downtime and incorrect installation.