Table of Contents
16 August 2023 Brake master cylinder: function, failure symptoms, and replacement
Master cylinders are used in both disc and drum brakes. They convert the pressure applied on the brake pedal into hydraulic braking power. This is done by the brake master cylinder feeding and controlling the brake fluid into the brake circuits, depending on the force exerted on the brake pedal. By efficiently converting the force applied to the brake pedal, it ensures that your vehicle comes to a safe and precise stop. Whether your vehicle is equipped with disc brakes or drum brakes, understanding the function and importance of the master cylinder is vital for automotive enthusiasts and safety-conscious drivers alike. Let’s take a look at this very important component.
Master cylinder on older vehicle models
On older models, the primary sleeve moves over the balancing bore, closing the pressure chamber and generating brake pressure. When the driver releases the brake pedal, the spring-loaded piston is pushed back to its original position. Car brake fluid then flows into the master cylinder. If the brakes are immediately applied again, the brake pressure is immediately increased.
Defective master cylinder symptoms
Reduced braking power or longer braking time is always an indication of a faulty master cylinder, as only two wheels are being braked instead of four. As one of the pressure chambers is no longer filled with brake fluid, the pressure point moves noticeably backwards and the pedal has to be depressed much further to feel any braking force.
A.B.S. Brake master cylinder
Number of connectors: 2, Aluminium, 1x M10x1.0
Article number: 81226
£ 44,29
price incl. 20% VAT, excl. delivery costs
-59%
TRW Brake master cylinder
Number of connectors: 2, D1: 25,5 mm, Aluminium, M12x1
Article number: PML138
compared to
RRP
£ 154,23
£ 61,72
price incl. 20% VAT, excl. delivery costs
ATE Brake master cylinder
Number of connectors: 2, D1: 25,4 mm, M12x1
Article number: 03.2025-2521.3
£ 80,48
price incl. 20% VAT, excl. delivery costs
-56%
Article number: 03.2126-3121.3
compared to
RRP
£ 202,80
£ 87,88
price incl. 20% VAT, excl. delivery costs
BOSCH Brake master cylinder
Number of connectors: 2, Piston Ø: 22,2 mm, Aluminium, M 10 x 1
Article number: 0 204 054 366
£ 87,16
price incl. 20% VAT, excl. delivery costs
A soft brake pedal, where the foot sinks towards the floor, is also a warning sign of a faulty brake master cylinder. At the same time, the brake pressure sensor will sound an alarm and the warning light on the trip computer will indicate that the component is faulty. In this case, you should drive at an appropriate speed or have the car towed to a garage.
When should you replace the brake master cylinder?
There is no real average service life for a master cylinder. That's why it's important to have it checked during a service or general inspection. However, there are certain signs that can indicate that it is malfunctioning. If you feel that your brake pedal is too soft, too hard or does not spring back properly, this may be due to a car brake fluid pressure problem caused by the cylinder not functioning properly. A defective brake master cylinder cannot convert the pressure applied to the brake pedal evenly to the wheels causing the vehicle to skid when braking.
If you notice a drop in brake fluid or traces of brake fluid on the master cylinder of your car during an inspection, this may indicate a leak. You should also examine the colour of the brake fluid in the cylinder glass. If it tends to be a blackish brown, you should bleed the brake fluid. As a general rule, the brake fluid should be changed every two years to prevent the inside of the cylinder from rusting. This may be the perfect time to take a look at our article Car brake fluid guide: check, how often to change and cost to get further information.
Article number: 6343
£ 66,54
price incl. 20% VAT, excl. delivery costs
BREMBO Brake master cylinder
Bore Ø: 22,2 mm, Cast Iron, 10 x 1 (4)
Article number: M 85 035
£ 44,18
price incl. 20% VAT, excl. delivery costs
DELPHI Brake master cylinder
Piston Ø: 23,8 mm, M10x1
Article number: LM80328
£ 48,88
price incl. 20% VAT, excl. delivery costs
BOSCH Brake master cylinder
Number of connectors: 3, Piston Ø: 23,81 mm, without brake fluid reservoir, Cast Iron, M 10 x 1
Article number: F 026 003 136
£ 38,92
price incl. 20% VAT, excl. delivery costs
Article number: LM42031
£ 22,96
price incl. 20% VAT, excl. delivery costs
Replacing the master cylinder
As the master brake cylinder is one of the car's safety-relevant components, it should be replaced at a specialist workshop using a car master cylinder repair kit. If the repair is carried out incorrectly, you could endanger not only yourself but also other road users. There is also a risk of damaging other components, such as the ABS unit, when replacing the master cylinder. The problem with replacing the cylinder is that the brake fluid and the valves and seals to the expansion reservoir have to be replaced as well.
Summary
The master cylinder is an essential component of any vehicle's braking system, acting as the heart that ensures the correct distribution of hydraulic pressure to each wheel. Its role in converting the force applied to the brake pedal into the vehicle's stopping power cannot be underestimated. As automotive engineering continues to evolve, we can expect further improvements in master cylinder design, resulting in even safer and more efficient braking systems for all drivers.
