The volume in the initial condition (Variables tab) is supposed to be the volume of the liquid. Not sure about R2013a, but in more recent versions, the I.C. is named "Volume of Liquid".
There are also two pressures: one in the parameter, which you have to specify; and one in the variables, which you may choose to specify. The former, pre-charge pressure, refers to "pressure in the gas chamber when the fluid chamber is empty" (see Help). Together with the "total accumulator volume" parameter, it defines the amount of air. There is also an initial pressure of the liquid, which should be the same as the initial air pressure, in the Variables (I.C.) tab. This defines how much the air is compressed from the precharge state. Since the amount of the air is fixed, setting the initial air pressure also implies a certain amount of liquid inside the chamber. As such, one should not set volume of fluid and pressure of fluids at the same time to High priority.