Hi Terry,
This is what is known as an accumulator, the Saracen and Saladin armoured vehicles also had three of them for steering and brakes. Another vehicle that had one was the Routemaster bus, so if you know anyone who owns one of these vehicles they will be able to help I would think. In the army they were charged with Dry Compressed Air, the Saracen was charged to 500 psi and suspect the Martian was the same. It is too many years I last worked on a Martian, I seem to recall the accumulator is under the cab on drivers side, and a bit awkward to access.. You can easily get Nitrogen to charge as a substitute.
Hope this is of help to you.
regards, Richard