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Your Brake master cylinder worn out? » Read more about common malfunctions • Repair • Replacement manual
At the head of the hydraulic braking system: the master brake cylinder
The master brake cylinder (master cylinder) is a central component of the braking system, converting the mechanical force of the brake pedal and the pneumatic force of the brake booster into hydraulic force which is transmitted to the wheel brake cylinders of the separate wheels without further loss. If master brake cylinder fails, the entire braking system is in danger.
Construction of a master brake cylinder
A master brake cylinder is a massive steel component with a piston rod. Internally it has five drilling holes. One of those accommodates the piston rod, allowing it to move in the cylinder. Four openings run sideways via external screw threads containing the brake lines. One or two more holes on the top are intended for the expansion tank of the brake fluid. The tank can be disengaged and mounted separately or alternatively integrated in the wheel brake system. Between piston rod and the tube of the master brake cylinder are several gaskets. These are the weak point in this component.
Function of a master brake cylinder
The master brake cylinder is screwed tightly in the engine compartment. Its piston rod is connected to the brake booster. When the brake is applied, the piston rod is pushed into the master brake cylinder with a force of 2 bar, putting the hydraulic fluid under pressure. This pressure is transmitted to the separate brake cylinders virtually without any loss of pressure. This functions on condition of no gas, air or water reaching the brake oil.
Defects of the master brake cylinder
A master brake cylinder has only one really weak point: the gaskets between piston rod and inside of the brake cylinder. If these fail, hydraulic oil can leak out sideways. This is perceived when braking pressure obviously deteriorates. The brake pedal has to be "pumped" to make it work. All in all the driving safety of a car with defective main brake cylinder is no longer guaranteed. The cylinder must be replaced as soon as possible.
Repair of the master brake cylinder
In the past, standard repair of master brake cylinder was a change of gaskets. Theoretically this is still an option. For reasons of guarantee as well as the cheap prices for this wear part, the master brake cylinder is now always completely replaced. This replacement can be somewhat tricky: there is a danger of air penetrating in the brake line. Therefore we recommend the following steps:
- Check the water level in the brake oil with a testing strip. If this level approaches the threshold value, drain the brake oil completely, replace the cylinder and fill the braking system with fresh braking fluid. By doing so, you perform a safe and full repair. Do not forget to vent the braking system following repair.
- Replacing master brake cylinder is very simple: the four connected brake lines are unscrewed. Then, the master brake cylinder is unscrewed from its mounting plate. Subsequently it can be pulled from the brake booster. A split pin or an extra nut may be applied for safety. The disassembly is very simple and can be performed by anyone with simple tools.
- We always recommend replacing the expansion tank simultaneously with the master brake cylinder. By doing so, you always have a new unit installed and don't need to worry about cracking or leaking of these components.
- Unscrewing the brake lines and the venting of the braking system are always an excellent occasion for inspecting lines and hoses. The lines must be rust-free. The hoses should not be brittle or porous. Possible biting marks of weasels can be found on these parts. "Ballooning" has become rare, but can occur in cheap brake hoses. Here too applies: everything that looks suspicious must be replaced. Otherwise even the best master brake cylinder is useless.
Costs for a new main brake cylinder
A master cylinder is a safety-relevant component and only the best quality should be installed. Inferior products from obscure manufacturers are always very risky. Expenses starting at 50 EUR (c. £45) for a high-quality master brake cylinder from a branded manufacturer are in no way excessive.