If your Brother printer suddenly runs out of toner and instantly stops printing, even though it printed perfectly well earlier in the day, you might be able to coax more out the cartridge by gently rocking it back and forth. But ultimately, the toner-out light will stay on and eventually the printer will run out of toner.
When this first happens the toner cartridge is probably not empty. The web reports most people consistently get another 800 to 1,000 pages by covering a sensor hole and tricking the printer to thinking that it is still full of toner dust.
I think the real reason is more benign. Measuring toner levels is more of an art than a science and without spending a ton of money on an interior electronics, Brother used a relatively simple but flawed mechanism to measure toner. Why did they put in extra toner? They wanted to ensure they reached their advertised page count with no chance of falling short. In other words, they over-filled the cartridge to make sure they were honest. But it still it seems wasteful not to use all of the available toner.
While you have your cartridge out of your printer why not also take the time to give it a gentle clean using a pipe cleaner. Avoid contact with the digital imaging drum if present but feel free to clean off any obvious leaks.
So remember that a toner cartridge has additional of life remaining even when the toner light turns on.