What makes brake strength fade with a speed control? I have a Tekin RS about three years old. Brakes were strong up till about three weeks ago, and now nothing. Am I missing something?

As your motor's rotor's magnetizm fades through either age or more often, having been ran too hot too often, your brakes will fade as well. Try a different motor and see if they come back. Or, you accidentally re-set the profile on your ESC to one with less brakes, double check that.