Brakes not working

This is for my brothers vehicle which I hope we can get some help on.
I was stopped at a red light today and all of a sudden my foot goes almost to the floor and the brake idiot light turns on. I make it home ok and check the brake fluid reservoir. It is not all the way to the top, but you can definitely see that there is some in there.

Any ideas what is going on here?

1999 4 door 4WD Chevy Blazer

Thanks
Steve

Top it off, pump it up, hold foot as far to floor as you can and have someone crawl underneath and check for leaks. Also check near the wheel/tires.

Other than crawling around in the cold snow garbage that was the way to find out.
Found the leak in the line under the driver side. Nobody seems to have a replacement line so will go with a universal fit.
Thanks
Steve