Brakes. Very 'spongy' very weak brakes, but fluid levels are fine.

As long as you've confirmed that you have fluid, you don't even need to replace the sensor.

Air is getting into the system hence the spongy feel to the pedal. The problem may lie with the brake assist system - have it checked professionally - brakes matter!

Do not drive the car. Check brake fluid level, if low fill. Spongy usually means air in the brake line. This requires bleeding the brakes.

air in the line or bad booster. try getting the brakes bled if it doesnt fix it probably a bad booster

find a good straight away and hammer the brakes. if brakes feel solid. check you brake fluid level at brake master cylinder engine bay, drivers side firewall. if fluid level is ok unplug the level sender at master cylinder resevoir-if this resets the light it is the switch in the master cylinder. If not unplug the switch attached to the e-brake lever.
if the brake feel spongy or fluid is low check for a leak in the hydraulic system.

I hope you have enough sense to stop driving it and stop added brake fluid

Lets review:

ABS Light is on--You may have an ABS System failure,do we continue to drive it, no
But you did,you said awhile
Spongy brakes--normal,I don't think so
Brakes go to floor--how many things can cause that,maybe 2
Emergency Light--now were in big trouble,no brake fluid
Added Brake Fluid---no fluids are consumed on a motor vehicle
any fluid loss is from a leak and requires repair
You never top off any fluid,don't need to

You have an older vehicle with rusted out brake lines
You need to buy brake tubing,cut,bend,flair and fabricate new ones
No vehicle in history, ever had brake lines made for replacement

Often this is because the brake fluid level is low. There is a sensor in the brake fluid reservoir that turns on the dash warning light when it does not sense enough fluid. You can visually check the brake fluid reservoir by looking at the reservoir under the hood. There are marks on the reservoir to show the correct level. If the fluid is low, this means that one of your brake system components is leaking fluid and needs to be replaced or repaired. If the fluid level is ok, possibly there could be air bubbles in the system, and it needs to be bled. If bleeding does not solve the problem, it could mean that the master cylinder is not holding pressure. If the brake pedal is spongy but it becomes firm after a few pumps, usually this means air in the system. If it stays spongy after pumping, and sinks continually when you press it, probably the master cylinder needs to be checked.

you need to find out why they are spongy? fluid low? brake fluid leak? master cylinder problem? ck the fluid level, if its real low, you need to find out where it went.

This is not just a matter of checking brake fluid level. Most likely you need to have the brake fluid renewed in the entire brake system, something you should do biannually or so to prevent trouble.

If done in a professional way it will cure your problem, this process includes venting the brake lines for gas bubbles that are trapped inside causing the spongy feeling.

While it is not very difficult to do yourself I would still recommend you to have a professional do it for you, not worth the risks of potential brake failure if done wrong. It is normally an inexpensive procedure.

the red brake warning light will usually indicate low brake pressure,abs light can come on also,check fluid level,does brake pedal become soft and spongy?if so it can be the brakes master cylinder,if fluid level is low,can be a damaged brake line,inspect brake system,