Brake pedal falling

There's no return spring on the brake pedal, and one shouldn't ever be needed. If you need one, something is wrong with the brakes. If the pedal is falling, you have a brake system issue. Probably at the master cylinder (or booster if power), bad seal, low fluid, leak, something along those lines. If the pedal doesn't return all the way after a brake application, it's more than likely a brake line issue, the fluid is being restricted in it's return. That's usually the rubber lines going out to the wheels, or the frame to axle line in the back. It can also be a sticky wheel cylinder or worn out brake shoe return springs. And if the pedal just sits low to the floor all the time, it applies and returns fine to the same location every time but to a spot which is lower than the pedal stop, it could be the master cylinder push rod is too short. But in that case the brake pedal would have ALWAYS been like that, it's not something that would just start happening. If it just started it's something else (line problems, leaks, bad booster, master, etc).

Regardless, you shouldn't add a spring, you should figure out what is wrong with your brakes.