Tell your friend to sit in the tractor's driver seat.
Locate the small nut behind one of the tires. Place a drain pan under the nut.
Tell your friend to push in the brake.
Unscrew the nut as your friend holds in the brake pedal. The brake fluid drains into the pan. Screw the nut back in once the fluid stops running out.
Perform Steps 3 and 4 on the other tire.