Brake light on the instrumentation panel on my Toyota Sienna 2004 is "on". The "foot brake" is not engaged. What should I do?

If your brake system seems to be operating ok, then more than likely the system is low on fluid. As brake pads wear, this allows greater travel in the caliper and thus the caliper piston has to travel further to make the pads come into contact with the brake rotor, causing the fluid level to drop in the reservoir. Once the fluid level drops so far, the brake warning lamp will illuminate. When you check the fluid level, also inspect under the vehicle and around the brake lines for and sign of brake fluid leakage that could be causing you to lose fluid as well.