Brake pedal goes to floor 2001 SE

I have a 2001 Grand Prix SE 3.1 that the brake pedal goes to the floor. I did notice the fluid was low , filled it up. I do not think it sucked air, although I can not be sure of that. As you can imagine, it is a beater so don't want to stick a ton of money in it. I had someone look at it that said he thought it was a "sensor" that would cost over 500 bucks. I don't know of any brake sensors at all. If there is, do they cost that much? If not would this be a master cylinder. Thanks for any and all help!

Sensors don't make brake pedals go to the floor.......either you have a bad master cylinder, you have a leak, or there is air in the system....if the fluid got that low you could have pumped air into the system.....

Also why did it get low? Due to a leak, or have brake pads/shoes worn down so much?

Just an FYI - it was a "simple fix". I had a good friend bleed the brakes. We found the right front brake line was bad. I got new brake lines and he replaced them. How ever, now the antilock brake light stays on. Is there a fix to this?

yes.....first thing is get the code....

To get the code I am sure you need a special tool for that. I don't have one. Are they universal? Where can I get then, any parts store? Thx for info!

You can rent an ABS scan tool form Autozone....find code...write it down....clear code....road test and see if the code comes back....if it does read code again.....