As you probably know the brake fluid level and handbrake warning are on the same circuit. Note there are two sets of brake fluid (and washer) sub looms to accommodate left and right hand drive cars. Black/white wires, all parallel to a single feed to a nine way connector (only 8 pins used) connecting to drivers LED display.
Conclusion is that if handbrake warning works ok then there is some short to ground on the brake fluid looms either the active one or duplicate, or the m/cylinder float switch contacts are failing. Remove cap and check for latter. As suggested above it could simply be your fluid level is marginal though.
