Master cylinders are a central component in the modern hydraulic braking systems. They ensure that brake power is distributed correctly to the front and rear of the vehicle. Back in the old days, two master cylinders with independent reservoirs were used; they also had two different adjustable pushrods that moved the piston to generate the required pressure for the brake system to act on the wheels. This design was robust, and engineers searched for new ways to produce braking power.
Tandem style master cylinders were introduced into the market in 1962 and are used in the majority of commercial cars nowadays. It consists of a single unit with two pistons inside, either with a single reservoir or two separated reservoirs to compensate for brake fluid loses. They use a single pushrod connected to the pedal, and this feature helps to reduce the weight and overall size of the system. The most important parts of the master cylinder assembly are the cup type seals, the springs, and residual pressure valves.
A simple approach to how brakes work goes as follows; When we press the brake pedal, the pushrod moves the pistons inside the master cylinder, and the seals are pushed against the piston face by the brake fluid as the clearances of the system are covered. The pressure starts to rise when all clearances are covered, and the car starts to brake. When pedal force is reduced or detained, the spring mechanically retracts the pushrod, and the braking force diminishes. There’s a chance for air to leak into the system, to prevent this from happening, residual pressure valves are installed in the cylinder, commonly at the front and rear outlets. These valves keep the system slightly pressurized, between 6 and 25 psi, and avoid air bubbles from forming.
Because force and movement are essential variables considered in brake systems performance, it’s a common thing that the symptoms of a bad brake master cylinder are related to the applied force and pedal displacement. This article intends to let you know the most common symptoms of a bad master cylinder.
The pedal has to travel more before starting to develop braking power
Feeling that your pedal is loose and the brake power seems misbalanced, meaning that the wheels suddenly lock, is a sign that the cup seals inside the cylinder are worn-out or they failed.
This problem is caused by pressing the pedal with a higher force than required, maybe because you had to brake unexpectedly and had no time to measure your strength.
Driving a car with worn-out seals is extremely dangerous due to the limited braking ability. But to be sure that the seals are the problem you can do a simple test with your car. The test consists of driving, preferably in an empty street and at low speeds, and pumping the brake pedal. As you pump, the pressure increases, and the car starts to develop braking power. If the brakes don’t work after pumping, then you might be facing a pushrod failure.
Abnormal consumption of contaminated brake fluid.
The brake system is constantly pressurized and depressurized and contains many seals located in the different components. Abnormal consumption of brake fluid, accompanied by a harder brake pedal, could be a symptom of excessive heat that causes the fluid to vaporize inside the brake line. If the fluid consumption is alarmingly high, and the brake pedal behavior is unaffected, then you must be facing a leakage caused by worn-out master cylinder seals. Constant pressure changes of the brake system cause the cup seals failure, mostly after long usage periods.
The best way to confirm this symptom is by visual inspection of the master cylinder, you’ll see that it’s drenched in brake fluid.
Another signal of damaged seals to be considered is the contamination of the brake fluid. Clean brake fluid is transparent or light yellow if we are going to change the brake fluid, and after purging, it turns out to be dark brown or black, maybe with suspended particles, the seals must be replaced.
The brake pedal feels spongy, and it slowly travels down after releasing it.
One of the worst things that could happen to your brake system is to have air bubbles inside the master cylinder. The brake fluid absorbs moisture, which can turn into air bubbles inside the system and cause problems. Air usually is caused by vaporization inside the brake line that enters the cylinder because of a malfunctioning residual pressure valve.
This symptom feels like a bounce-back movement of the pedal and, after stopping and releasing the pedal, it travels back a considerable distance before resting.
It is crucial to keep air outside the cylinder because it damages the seals and causes more air to enter, and it damages the brake hoses, causing the whole brake system to fail.
Engine light comes on.
The check engine light will turn on if any of the car’s sensors read an off-range value. To know which of the sensors is causing the problem, you need to run a computer scan of your car using an OBD reader. Master cylinders use pressure sensors to read off-design parameters, which could cause your brake system to collapse. This symptom is the most trivial since you can’t relate the light directly with a problem; it could be any of the sensors from the many systems that ensure the normal functioning of your car.
For closure, you could’ve noticed that springs are not mentioned in any of the symptoms of a bad master cylinder, simply because since they’re inside the cylinder, which works as a guide and prevents buckling caused by the acting compressive force. A rare condition could happen when the spring snaps out of its attachment point, and this is one of the primary diagnoses when the pedal won’t return completely into position after releasing it. It’s not a common failure, but it’s important to mention it.