CONTAMINATED BRAKE FLUID.
Even though your brakes operate in a closed system, contaminants can still work there way into the system. Air can enter the system through the smallest hole, and you can end up with water in the system from condensation and other means. There’s not really any way to check for this, but bleeding your brakes will remove the bad stuff and replace it with new fluid.
WORN BRAKE PADS.
Your brakes should never wear low enough to cause your brake pedal to feel low, they’ll scream at you before then. But if they do get very low, you might have this problem. Replace your brake pads as soon as possible. Of course, this can be avoided with regular brake inspection.
BAD BRAKE POWER BOOST UNIT
Finally, if your brake booster goes bad you’ll have low brake pedal issues. Most brake boosters are vacuum controlled, so a special vacuum measurement device that connects to the brake booster is needed to check it. If it’s bad, you’ll have to replace the boost unit.