This Bosch master brake cylinder is a direct-fit replacement part designed for selected Citroën and Peugeot models. Engineered to convert pedal force into hydraulic pressure, the unit works together with the brake booster and the rest of the hydraulic system to deliver reliable, progressive braking response. The part is commonly sought under reference numbers 0204123596, 204123596, 22472579 and 4601L2, so you will often find it by catalog code when ordering or searching online.

Technical Information

• Manufacturer: Bosch
• Model(s): Citroën, Peugeot (e.g. Peugeot 307, Peugeot 308, Citroën C4 – see product codes for fitment)
• Product Codes: 0204123596, 204123596
• Additional Numbers: 22472579, 4601L2

Function And Application

The master brake cylinder is a central component of the hydraulic brake system. When the driver depresses the brake pedal, the piston(s) inside the master cylinder pressurize brake fluid, which is then transmitted through brake lines to the calipers or wheel cylinders. A properly functioning master cylinder ensures consistent pedal feel, correct pressure distribution between front and rear circuits, and safe braking performance.

How To Replace (Overview)

Replacement is a moderate-level job suitable for experienced DIYers and professional technicians. Typical steps are:

• Park vehicle on a level surface and secure wheels. Wear eye protection and gloves.
• Remove brake fluid from reservoir or siphon to avoid spills; keep system as clean as possible.
• Carefully disconnect brake lines with a proper flare-nut wrench to avoid rounding fittings; plug lines to prevent contamination and fluid loss.
• Unbolt the master cylinder from the brake booster and remove the unit. Some applications require transfer of the fluid reservoir or use of new sealing gaskets.
• Install new master cylinder, tighten fasteners to manufacturer torque, reconnect brake lines and ensure all fittings are tight.
• Completely bleed the brake system (recommended sequence per manufacturer) until no air remains and correct pedal firmness is achieved. Top up with DOT 4 fluid as specified by the vehicle maker.
• Check for leaks, road-test at low speed and re-check lines and fluid level after initial use.

Mounting Recommendation

Always follow the vehicle manufacturer’s service manual. Use new sealing washers on hydraulic fittings and replace any rubber seals that look aged. Use a proper line wrench to avoid damage to fittings, and always bleed the system thoroughly after installation. For safety-critical work like brakes, consider installation by a professional mechanic if you are not fully experienced. Compatible brake fluid for most Citroën/Peugeot applications is DOT 4 — verify fluid specification for your specific model before use.

Reason Why Part Most Often Fails

Master cylinder failures are most commonly caused by internal seal deterioration and contamination of brake fluid. Over time brake fluid absorbs moisture, which accelerates corrosion of internal components and causes rubber seals to swell or crack. Contaminated fluid carrying debris also abrades sealing surfaces. External leaks at line fittings or reservoir connections, as well as prolonged exposure to heat and lack of regular brake-fluid changes, are frequent contributors. Symptoms of a failing master cylinder include a spongy or sinking brake pedal, visible fluid leaks, uneven braking or a brake warning light.

Maintenance Tips

• Change brake fluid according to manufacturer intervals (commonly every 1–2 years) to reduce moisture and corrosion risk.
• Inspect for external leaks and check pedal feel periodically.
• When replacing the master cylinder, always bleed the system properly and verify there are no air pockets remaining.