There is no "motor" in that hydraulic accumulator. It only stores a limited amount of hydraulic oil pressure by compressing a spring in the accumulator when the truck engine runs the hydraulic pump. The hydraulic pump pressurizes the accumulator and the accumulator stores the energy until it is needed. The accumulator assists the brakes at times when there may not be enough hydraulic pump pressure available, like at idle when you are also operating some hydraulic cylinders. The accumulator only has enough energy stored to affect a few brake applications before being depleted.
Once depleted you have no power assisted brakes. The brake application then becomes a matter of how hard can you push on the brake pedal because at that time, the brake valve has switched over to being a master cylinder to provide back up for the brake system. There is no back up brake booster pump.