Brake pedal goes all the way to the floor on stop lights. when new brake pads were installed?

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.