A leak or a failed master cylinder.
Master cylinder may be leaking, brake pedal may slowly fall to the floor when you are stopped with your foot on the brake.
usually they will leak out the back by the booster, your brake pedal will slowly go to the floor at stops or the brake pedal will go right to the floor when pushed.
When you press and hold the brake pedal it will slowly move to the floor. It can also have a catastrophic failure and go straight to the floor with no stopping ability at all. If you hold the brake pedal down and it slowly moves to the floor any at all, replace the master cylinder.
possibly needs a master cylinder
Although they can fail abruptly the most common symptom you normally see is that when you stop and hold pressure on the brake pedal it will very slowly go down toward the floor. It will not hold steady pressure because the seals are bad. It must be replaced immediately.
First, make sure the master cylinder is filled with brake fluid. If it is low, there may be a leak somewhere in the brake lines or a brake cylinder. If the master cylinder is bad, you can normally pump the brakes up, but the pedal will slowly sink to the floor again. The master cylinder needs to be replaced or repaired.
Check your brake fluid. Next maybe your brake lines need to be bled. OR you just might need new brakes.
The most common way to test this is,at a stop with your foot on the brake, the pedal will very slowly go to the floor, there is a test for this but you need to be good at auto repair.
When you hold pressure on the brake pedal at a stop light and the pedal slowly drops to the floor and the brake fluid reservoir still remains full, the master is bad.
chek the brake fluid level at the master cylinder, if it is empty, then you have a probable leak and \or air in lines. If it is still full, then it's probably the seals on the piston inside the master brake cylinder.
A symptom you almost always feel is a brake pedal that will not hold. You stop, hold the brake, and the pedal will slowly move toward the floor. This is a sure sign the master cylinder is bad. A leaking master cylinder or one that sticks, is also a sign is is defective. When you replace the master cylinder replace all the brake fluid in the brake system and bleed the brakes. Moisture in the brake fluid will destroy a master cylinder and this may be why it failed.Brake fluid leaking from the master cylinder. When you stop and hold your foot on the break the brake pedal will slowly go to the floor. Either of these symptoms means your master cylinder must be replaced.
As with any other vehicle, this would indicate either a brake fluid leak, an empty brake fluid reservoir, or the failure of the brake master cylinder piston seals. It should go without saying that the vehicle in question should not be driven until the brakes are completely and properly repaired.