handbrake accumulator?
is this an air operated brake? or manual?
if air operated then:

has the brake system been drained of moisture lately? This could be the result of a gunked up sensor on the accumulator sensor causing it to set the code.
A common problem with air brake systems if they are not maintained properly.

Pull out the sensor and see if it is gummed up and bleed the water out of the air tank. You may have some clean up to do in the air lines and components affected as well.

if manual operated then it could just be a switch adjustment