#1 – Contaminated brake fluid
The rubber seals on the brake master cylinder wear out over time. If the rubber seals are in poor shape, the brake fluid can become contaminated and turn a darker color.
Another effect of worn rubber seals is the inability of the master cylinder to withstand the brake pressure applied to it. As a result, the brake pedal will feel mushy and start to slowly fall to the floor.
#2 – Abnormal brake pedal
One of the first things you'll notice when the master cylinder fails is the brake pedal is acting abnormally. After all, the brake pressure of the brake system comes from the master cylinder. If it doesn't handle or seal this pressure as it normally would, then you'll feel the brake pedal abnormally.
Back to the rubber seals on the master cylinder, if they wear out, they can cause internal leaks of brake fluid. Once this happens, the pedals feel mushy and gradually fall to the floor after you take your foot off the pedals.
#3 – Low brake fluid
If the brake fluid level in the brake master cylinder is low, you may experience some braking problems. The first thing you'll notice is that the brake pedal feels soft when you lift your foot.
If there are no fluid leaks and the brake fluid level appears normal, your master cylinder may be the culprit. The brake fluid in the reservoir needs to be sealed there, and any hose connections need to be tight and secure.
If you notice a leak in your brake master cylinder and you're sure it's not a loose hose, a cracked reservoir, or a leaking cap, replace the brake cylinder as soon as possible.
#4 – Check if the engine light is on
Many newer cars have master cylinders with built-in pressure sensors and brake fluid sensors. These sensors will detect if there is a problem with the vehicle's brake fluid pressure.
For example, if a drop in pressure is found, which could be caused by the master cylinder, the car's ECU will log a diagnostic trouble code, which will trigger a "check engine" warning light on the dash.
If this light comes on, it may be because of the master cylinder. Of course, you won't know the exact reason for checking the engine light until you scan the car.
However, if the light comes along with any of the other symptoms here, chances are it's your master tank. Using an OBD2 scanner is the only way to confirm which error codes are stored.