Q: brake pedal goes all the way down

I am assuming that once the brake pedal engages the brakes, that the car stops ok. One thing in particular comes to mind and that is seized or binding caliper slides. One piston calipers have to be mounted on slides. The piston pushes on the inboard pad, that in turn pulls the caliper inward, pulling on the outside pad. When the slides don’t move, there is enough hydraulic pressure when applying the brake to force, flex, and bend parts until the pads clamp the rotor, but when they flex back, that pushes the piston back into the caliper. That is what is going on in the pedal travel that is soft. It could be one caliper, or all four. The extra distance created by the piston being pushed back in, is the extra distance of the pedal. Pads seized in their anchors can have the same effect.

I have seen the piston seal in the caliper pull the piston back as well. The brakes would feel fine when new, but as the pads wore down, the distance needed for the piston to travel would become greater and greater.

Parts not moving freely can cause other problems such as the pads "tapering". That is when one area of the pad is thicker, and it tapers to being thinner. It can be subtle. The pad can taper top to bottom, or long ways. This by itself can cause a slightly low pedal, and both problems together are cumulative. Measure all areas of the pad to check for this. 2mm or 2/32" max taper.

You have one-piston calipers on the front. The rear could have one or two piston calipers. Two piston calipers do not need slides and are bolted rigidly in place. They have one piston on each side of the rotor. Check both.

If the slides are to blame, the usually can be taken apart, cleaned and greased to work properly again. Same with the pads anchors. However, if anything is badly corroded, the caliper(s) will need replaced. If the pads are out of spec, worn out or close to it, they’ll need replaced as well. If the rotors are out of spec, or badly rusted, you guessed it, they’ll need replaced. Calipers should be replaced in pairs to prevent the car pulling in one direction when braking.

Conclusion: Have your brakes inspected by a professional. Have them check for seized or binding slides or pads. Check the pads and rotors for unusual wear. Check the caliper pistons to make sure they slide on the seal as they should. Check the condition of the fluid. Brown or black fluid needs flushed out. But I suggest having a mechanic inspect it first to find out just what you need.