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The brake pedal will sink to the floor when applied and the brake fluid reservoir remains full.
You will be adding brake fluid to the reservoir often.The brake pedal won't feel firm.The brake pedal may sink to the floor as you are applying steady pressure.Do a visual inspection of the brake lines looking for that wet spot.
A brake fluid leak in the system would do that. Check the brake fluid reservoir. If it is empty you are in need of brake work asap!
Probable causes: Low or no fluid in reservoir; Worn brake pads/shoes; Air in brake lines; Leaking brake line.
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.
Try to pump it to build up the pressure.
There are several reasons why your car brake pedal can go to the floor. None of them are good. Your reservoir might be low on brake fluid. Your master cylinder might be worn out. Some of the brake cylinders might be worn out. Whatever it is it is time to get it fixed before you kill someone else and perhaps yourself.
If the master cylinder reservoir is empty, you have a leak somewhere. If the master cylinder reservoir is full, the master cylinder is bad.
If it's not on top of the brake pedal look on the brake reservoir.
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.
if the vehicle has a low brake pedal, check for brake fluid leaks, or out of adjustment brakes. if it is just low on fluid, check for worn out brake linings.
Chek your brake fluid level. There should be a reservoir under the hood, close to the firewall.