I have a 2003 Disco II
The brake light on the dash is always on.
I have replaced the cap on the fluid reservoir.
What else do I need to check?

 

parking brake switch is the next thing to check.... but based on your other post about the three amigos.... it's probably related. The light can come on if your electronic brake distribution system is detecting a fault. Probably that sensor or hub you've got issues with.

 

My opinion on this would be that during a recent pad change someone forced the pistons in to fast or without opening the bleeder causing old dirty fluid to re-enter the system & crud everything up. That being said you need to flush out the " old dirty " and re-bleed the system if your not 1000% confident to do this task then don't. Please don't experiment with the braking system on this heavy SUV you could kill yourself or others. Take to a professional brake shop or Indy shop. Seems that your having a lot of issues with your truck and you need to either take this to a Indy shop or slow down and focus on one issue at a time and exchange information with us promptly so we can help you get your truck sorted out.