I have a new to me Dyson DC25 Animal. The brush bar will not engage unless I press and hold the grey brush button. If I hold down the button while vacuuming the brush bar operates. Once I release the button the bar stops. I looked for frayed wires on the lower side by the ball. I am a white boxer owner and really like the ability of the DC25 to keep those short dog hair from taking over my house! Any help on fixing this problem would be greatly appreciated.