I was descending a long, steep hill yesterday and could feel the back brake lever getting perilously close to the grip, making me perilously close to crashing. I could only supply so much brake to the front wheel.
I used to have a car where the brake pedal slowly went to the floor unless you pumped the brakes periodically. So I pumped the brake lever and just like that the brake handle returned to being a reasonable distance from the grip, which is why I am not all scraped up today.
But as I kept squeezing the brake handle, it would slowly sag toward the grip and I'd have to pump it. Here's a pic without pumping: