Here is a pic of the type of master cylinder I have (mine has the filler tube and dual vacuum connections):
Yes, the vacuum reservoir does look like a scuba tank. The solenoid is attached to the reservoir by a short piece of vacuum tube, and has a ground wire and a hot wire connection. When I took the solenoid apart, it has a rubber stopper which, when activated, pushes against the inner port and blocks air flow. The brake pedal does say POWER BRAKE.