Answer check the brake fluid, it might be low. Answer First make sure the master is full of brake fluid and no loss of fluid. If you have drums in the back, did you adjust them properly, spin drum by hand and adjust till they are a little tight. Next, if all that is good, it is possible that the brake master is by-passing. For that, you have to replace the master. Make sure to bench bleed first.
If the brake pedal sinks to the floor while you're sitting at a red light, the master is by-passing as allready stated above. Replace the master. Bench bleed the master before you install.
The brake light switch is out of alligment. Check on the brake pedal arm and readjust the switch.
Check the brake switch up under the dash on the brake pedal. Lie on the drivers floor, look up and then push the brake pedal with yoru hand and see if there is any kind of electrical connector that actuates the brake lights when you push the pedal in.
Sounds like the brake light switch is broken. It is located above the brake pedal, replace it with a new one. Your local auto parts store will have that part.
brake light switch, may be located above the brake pedal where it mounts under the dash, did you find any parts laying on the floor ? No I did not
no you have a brake fluid leak somewere
Get down on the floor board and look up at the brake pedal arm. You will see a switch that the arm hits when no pressure is applied to the brake. Twist the switch and adjust so when you barely touch the pedal the brake lights come on.
Through on the floor
There should be a bracket mounted on the steering column near the floor. This will have a switch in it that is a plunger type. When the shaft of the brake pedal is depressed, it comes into contact with this plunger and activates the brake light circuit. If you "ride the brakes", your brake lights will always be on because of the minimal pressure required to activate this circuit.
In the floor cover behind the drivers seat (turn the knob on the cover & lift it out) you'll see some inline fuse holders in the wiring harness........... one of them is the trailer brake lights Good Luck !!!
If you have already checked the brake fluid levels, you'll have to replace the master cylinder.
replace brake booster
Get down on the floor board and look up at the brake pedal arm. You will see a switch that the arm hits when no pressure is applied to the brake. Twist the switch and pull out of bracket and then remove wires. Do reverse when installing new switch adjust so when you barely touch the pedal the brake lights come on.