Brake pumps: what they are and how they work

The brake master cylinder is the heart of a vehicle's hydraulic braking system. Its main function is to convert the mechanical pressure applied by the driver to the pedal into hydraulic pressure, forcing brake fluid through the lines to the wheel calipers or drums.

How It Works

Its operation is based on Pascal's principle, which states that pressure exerted on a liquid is transmitted uniformly in all directions.

1. Action: When you press the pedal, a push rod moves one or more pistons inside the pump's cylindrical body.
2. Pressure: This action compresses the brake fluid, forcing it into the hydraulic circuit.
3. Braking: The pressurized fluid reaches the caliper pistons, which push the brake pads against the discs, slowing the vehicle down through friction.

Main Components

• Pump body (Cylinder): The housing that contains the liquid and the mechanism.
• Tank: A reservoir located above the pump that holds the brake fluid supply.
• Piston and Seals (Rubber Seals): They provide the necessary seal to generate pressure without internal leaks.
• Return spring: Return the piston to its original position when you release the pedal.

Symptoms of Malfunction

If the pump is worn out or damaged (often due to deteriorated internal seals), you may notice:

• “Spongy” pedal: The pedal offers little resistance and feels “spongy.”
• The pedal sinks: When pressed with constant force, the pedal slowly lowers toward the floor of the car.
• Fluid leakage: Traces of brake fluid near the brake booster or inside the passenger compartment.
• Warning light on: The brake light coming on on the dashboard.

To ensure maximum safety, it is essential to bleed the air from the system every time work is performed on the pump or the hydraulic circuit.

How to Measure a Brake Master Cylinder

To correctly identify a brake master cylinder and find a compatible replacement, it is important to take a few precise measurements. First, after removing the dust boot and circlip, use a caliper to measure the internal diameter of the cylinder, where the piston moves. If the piston does not come out easily, you can push it out with a suitable tool, taking care as the spring may cause it to pop out suddenly. Watch the tutorial at the end of the article to learn more.