Now that it's starting to get cold, my new-to-me (June) 2000 Accord V6 has been having the BRAKE light come on in the morning.

Stage 1:It will stay on constantly for a few minutes.
Stage 2: After driving for a few minutes, it will go off when I accelerate. Not when I let go of the brake pedal, but when I accelerate. Then when I start to decelerate or brake, the light will come on again
Stage 3:About the time I arrive at work (20 minute drive) the light is usually off completely, unless it is really cold, then maybe later in the day it will go off.

My code reader isn't picking up any codes, and I can't replicate the problem unless it's cold and the car has sat out all night. Also, the fluid level is good in the reservoir. Any ideas what's causing this, and how to fix it? TIA

 

Iwould suggest that you remove the brake fluid reservoir capand visually check the fluid level, the stain mark on the reservoir might make it look full; while the cap is off you can flip it upside down and start the car to be sure the brake light goes off, then flip it back over to see the brake light go on. that will verify the float andswitch is working properly.
You can also jumper the float switch plug as a troubleshooting method to see if the symptom changes.

 

this is a pretty common problem - the brake master cylinder filter creates too good a seal. next time it's happening open the hood, and (very quickly) remove the brake master cylinder cap and look into the reservoir, into the filter. see the fluid low or not even in the filter center?? there is a pressure difference inside reservoir. after removing the cap the pressure will equalize and level out. you have to modify your filter or buy the new updated filter from the dealer.