Yes, DON'T try to unscrew the bolt on top, and don't try to open it. It's bolted in place from the bottom. You can see the bolt in the picture(#4) that secures it.

The pressure is contained in the cylinder. It kind of acts like a shock absorber in the ABS. When the ABS pumps cycle, they run the pressurized brake fluid against the piston inside the accumulator, creating the system pressure.