Brake pedal almost touching floor

That's what it was. Bought a new (as in, not rebuilt) master cylinder at AutoZone for $43.99

Thank you Docfixit

Almost a year later, just 2 weeks short of an "anniversary, " I have little to no brake pedal. Same as before, I had to "plan" my stops further ahead of time on a slow trip home. This time however, I also have an ABS warning light that stays illuminated. The light and the poor brake pedal event are not exactly in sync, I started seeing this light maybe a week or so beforehand. I mention this only for being complete with my symptoms, as it may or may not be directly related.

I went straight to the master cylinder this time. I just replaced it, bench bleeding the new one and installing it. Then did the ritual bleeding with an assistant pushing and holding the brake pedal. When the car is off, there is plenty of firm brake pedal, but when the car is started the pedal goes way down, nearly touching the floor. So I tried bleeding some more, the master cylinder itself and also at each of the wheels. Still no luck. When bleeding, there was little to no fluid, suggesting no air in the lines.

Someone at the parts store suggested the other culprit could be the brake booster. Is replacing the booster the next logical step? I want to be sure there isn't anything else I can try before having to crawl under the dash.

To summarize, the brake pedal goes way down when the car is started/running. I replaced the master cylinder. There is enough brake to stop the car if I put it in gear, and let it idle forward/backward in my driveway. I also have the ABS light that stays on.

*** UPDATE ***
I think I found the likely problem. After noticing the brake fluid reservoir went down, I found a leak under the car, as shown in the photo. But I don't know what that rusty thing is. Does anyone know? Thanks for any help.