The new hydroboost unit bolts to the firewall. The master cylinder then bolts to the end of it. You can see the inlet and outlet hydraulic fluid ports that will be plumbed. The blue anodized can is an accumulator or nitrogen-reserve bottle which stores hydraulic power-steering pressure should the engine stall. This allows you to continue to have power assist with a stalled vehicle for about three braking applications of the pedal. After this reserve depletes, the brake behaves like a normal unassisted brake system until the engine restarts, and the accumulator is refilled.