can it be caused by dirty brake fluid that has to be flushed? the mechanic recommended that i needed to change it, and was just curious? bad fluid = brake light??

 

low fluid will cause brake light.

 

if the brake fluid is more than 3 years old, its time to do a brake fluid flush- many of us do it every year!
BUT
Its very unlikely to cause a warning light unless its low on fluid- which means the pads are low or there is a leak of fluid somewhere
The caps have been known to fail at its sliding sender to tell it ifs low on fluid
Check that sendor moves freely