The master cylinder pressurizes the brake system when the pedal is pressed or applied.  It also provides hydraulic fluid to the wheels. A red brake-warning light typically lights up with hydraulic failure.

When your vehicle is turned off, the brake pedal should remain firm. However, when you push the pedal lightly and it sinks to the floor, the master cylinder has a leak. When driving, if the brake pedal is fully depressed and feels unresponsive or spongy, this is also the result of a malfunctioning master cylinder.

It is often necessary to replace the master cylinder and bleed the hydraulic system of air.  You will want to get your vehicle repaired with a qualified auto technician as soon as possible due to the high safety factor involved.

a typical master cylinder

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