Your brakes dont work.

You need to replace it ... or overhaul it with a master cylinder kit ... if you are mechanically minded it's not that hard to do ... some parts shops will do an exchange unit ... you trade yours for an overhauled or new one

You will have no brakes.

your brakes will act as if the brake lines been cut though normally the first sign is brake fluid leak down the back of the master cylinder before the seals/pistons totally go.

I think I need to set the record straight here... First, let me explain what the master cylinder does quickly. The master cylinder contains a volume of brake fluid, has a cap, and (depending on the brake system setup) has two or more valves. The valves are the important parts as far as it functioning correctly. When you press on the brake pedal, vacuum pressure moves the valves to allow brake fluid to enter the brake lines. The brake fluid compresses the pistons in the calipers (disc brakes) and slows the vehicle. When you let off the brake pedal the fluid returns to the cylinder. When the master cylinder doesn't work, the usual culprit is the valves (providing the brake fluid has been maintained). Therefore, brake fluid is unable (or restricted in) entering the brake lines and braking power is substantially reduced. When brake lines burst, the brake pedal goes to the floor since there is no resistance as far as brake fluid in the lines. When the master cylinder fails (at least in my experience) the brake pedal does not go down as usual. It will feel like it is stuck. A normal driver will immediately gear down, turn on the hazards, and prepare to work the emergency brake for stopping. The vehicle should pull over as soon as is safe. When a master cylinder fails it should be replaced. Hope this helps, Good luck

I know this from experience. Your braking power will be reduced from 100 percent to 10 percent.

you lose control of bodily fuctions and rearend the poor bastard in front of you!

In brake testing the most common Master Cylinder failure we see is a leaking seal. This creates a pedal that slowly sinks to the floor when you hold it down. This also creates a differential pressure in the Primary and secondary circuit that will trigger a switch in the Proportioning valve (old school) or HCU (ABS new school) that turns on the red light. If left unrepaired this will eventually lead to a total failure of one of the two pressure circuits. This will turn off half of the brakes and leave you with a very long pedal and very reduced stopping power. Automotive companies no longer equip their vehicles with an Emergency Brake, They install a Parking Brake. The parking brake is just for that, parking. Using it to stop the vehicle while moving is dangerous because it works on the rear wheels only and can cause the vehicle to spin and possibly roll over. There are no federal tests required for stopping the vehicle with the parking brake.