What's one of the scariest things that can happen while driving a car. How about No Brakes! Brake failure can occur without warning. You stomp down on the brake pedal to apply the brakes and nothing is there. The pedal goes all the way to the floor and there are no brakes.
What can you do? Scream? Pray? Try pumping the brake pedal as fast as you can. It might generate enough pressure to apply the brakes and stop your car. If nothing happens, apply the parking (emergency) brake as hard as you can. It should start to slow your vehicle immediately. If you can't react quickly enough and are in danger of crashing into another vehicle, look for a way to avoid the collision. Blare your horn and try to steer your way around other vehicles or obstacles. Eventually your vehicle will coast to a stop. Pull over to the side of the road, shut the engine off, put the transmission into Park (or leave it in gear if it is a stick), set the parking brake and call for help. DO NOT attempt to drive your car until the problem that caused your brakes to fail has been diagnosed and repaired.
One of the most likely causes of no brakes is loss of fluid pressure in your brake system. The brakes operate using hydraulic pressure, so if there is a fluid leak in a brake line, brake hose, wheel cylinder or caliper, there may not be enough fluid pressure in the lines to apply the brakes.
When a serious leak occurs, the red BRAKE Warning Light should come on when you apply the brakes. This is to warn you that that has been a loss of pressure in part of your brake system and that your vehicle may be unsafe to drive in its present condition. Check the fluid level in the master cylinder reservoir. If the reservoir is extremely low or dry, you have a serious brake fluid leak. The entire brake system needs to be inspected for leaks so the fluid leak can be found the fixed.
Sudden fluid loss can occur if a rubber brake hose cracks or breaks, or if a steel brake line rusts through. Rusty steel brake lines are common on older vehicles that are exposed to lots of road salt and moisture. Salt can be very corrosive, especially if the anti-corrosion coating on the steel brake lines is thin or of poor quality. Once the rust eats through the line, you have a brake line failure as the fluid blows out and you lose your brakes.
If the brake pedal goes all the way to the floor when you apply the brakes, another cause might be severely worn brake linings. Or, air in your brake lines, which would require bleeding the lines to get rid of the air.
Another possible cause of brake failure might be a faulty ABS modulator that is leaking brake pressure internally and is not routing pressure to the brakes when you step on the pedal. This has been a known problem on older Ford and GM pickup trucks with rear-wheel Kelsey-Hayes antilock brakes. Dirt or rust in the brake system can enter the modulator and prevent the spring-loaded accumulator valve from closing, allowing the modular to leak internally.
Another possibility would be a bad Master Brake Cylinder. If the piston seals inside the master cylinder are worn or damaged and are not applying pressure when you push on the brake pedal.
A faulty power brake booster that is not providing much if any power assisted braking will increase pedal effort, but it will not cause the brakes to fail. You just have to push on the pedal much harder than normal to stop your car.
Brake fade and/or brake fluid boil are other conditions that may occur as a result of the brakes getting too hot (often as a result of mountain driving, driving aggressively, racing or riding the brakes for a long period of time. As heat builds up in the brake linings, it takes more and more pressure to achieve the same friction and braking force when the brakes are applied. The pedal may remain firm but the brakes just don't seem to have much stopping power when you apply them. The fix for this condition is to slow down and give the brakes a chance to cool. Hot brakes may also cause the fluid inside the front calipers to boil. This forms a steam pocket that may increase pedal travel to the point where there is not enough pedal travel to apply the brakes. Pumping the brakes may help in this situation.