The brake master cylinder pushes the brake fluid out of the brake fluid reservoir and into the brake lines. When the brake pedal is pushed in, the brake fluid leaves the reservoir and flows into the brake calipers. The fluid then pressurises the piston inside of the brake caliper, which in turn compresses the brake pads against the brake rotors. Inspect the brake master cylinder and the brake lines on a consistent basis for brake fluid leaks. If any brake fluid leaks are found, repair them immediately.
Park the vehicle on a level surface and open the bonnet. Have the helper raise the hood and lock it in place.
Leave the engine running and slowly push the brake pedal in and out a few times. Press the brake pedal in again until it stops moving and hold sustained pressure on the pedal. If the brake pedal stops moving and stabilises in one spot, the master cylinder is working properly. If the brake pedal continues to drop down while you are holding pressure on it, the master cylinder is either leaking or there is a brake fluid leak somewhere in the braking system.
Continue holding pressure on the brake pedal. Instruct the helper to remove the lid from the brake fluid reservoir to ensure that the brake fluid level is on the full mark. Reinstall the cap back onto the brake fluid reservoir.
Instruct the helper to inspect the brake fluid lines that are attached to the master cylinder for leaks. Leaks can occur from the threaded end of the brake lines that screw into the master cylinder. If no leaks are visible on the brake lines, instruct the helper to inspect the entire brake line from the brake fluid reservoir to the backside of the brake calipers behind each wheel. Use a flashlight if necessary.
Inspect the rubber grommets around the master cylinder for signs of leaking brake fluid. If there are signs of brake fluid leaking around the rubber grommets, this will mean that there is an eternal leak inside of the master cylinder and that the brake fluid is bypassing the O-rings.
Release the brake pedal and push it down again until the pedal stops moving. Have the helper remove the brake fluid reservoir lid again and inspect the brake fluid for bubbles. Bubbles in the brake fluid are a sign of air in the braking system, which can happen if the master cylinder is faulty.
If the brake pedal pushes all the way to the floor without stopping and there are no visible signs of brake fluid leaks, the brake master cylinder is faulty.
Always use caution when working around an engine that is running.