If the brake pedal goes to the floor but still stops, first is to check brake fluid level.
Open the hood of the vehicle and locate the brake fluid reservoir; it is usually sitting on top of the master cylinder. While looking at it, rock the vehicle slightly in order to see where the fluid level is at. Do not remove the reservoir cap for this.
If the brake fluid level is low then the system is lack of brake fluid as indicated above, the brake fluid level appeared low. When such a condition occurs, the brake pedal will travel further before it stops. In general, the lower the brake fluid level, the lower the pedal will travel.
What to do?
First, you should ask yourself, "How did the brake fluid level become low?" Could there be a leak somewhere in the brake system? We definitely recommend that you have this checked out. Or, were the brakes recently serviced and possibly the mechanic did not add enough brake fluid? In any case, you should add some brake fluid. Simply unscrew the top of the reservoir and carefully pour in the appropriate amount. Do not keep the cap off any longer than necessary.
If the brake fluid level low is not low next is to check the rear brake. If the vehicle rear brake is a drum brake then a worn brake shoe lining is causing the brake pedal to goes down the floor. A drum brake system basically involves brake shoes that press against the inside of a drum when the brake pedal is depressed. If the brake shoe linings are worn, the brake pedal will have to be pressed down further before the shoe linings come in contact with the drum.
What to do?
You'll need to take this vehicle into a shop and have the mechanic verify that the rear brake shoe linings are indeed worn and in need of replacement. As a general rule, only replace riveted linings when they are worn to 1/32" or less above the rivets. In the case of bonded linings, only replace them if the lining thickness is 1/16" or less. The mechanic may insist that he must machine the drums before he installs the new linings. Usually, this is not a bad idea and you should strongly consider having it done.