If I am understanding what you are talking about correctly, in my line of work a hydraulic press would be considered a full hydraulic system that has its power source derived from a electrically driven hydraulic pump and uses a mechanical hand operated control lever to vary the speed and direction of the ram assembly.
A hydro pneumatic would have an air powered reciprocating piston pump for the power source and could have either the direct hand operated control valve or a remotely controlled valve that uses air pressures to transfer the hand lever motions to the actual hydraulic control valve. The primary work medium is still hydraulic fluid but its using an air based power source to generate the pressure.
The hydraulic system works on the hydraulic oil or other liquid.
Whereas the hydropneumatic system works on air (or gaseous fluids - like Nitrogen mostly, or sometimes others too).
The basic difference is in compressibility.
Consider a cylinder, when a given quantity of fluid is given to it, it moves by that amount in a liquid system (they are usually considered incompressible)
However in a gaseous syetem, being compressible, the movement is not equal and much lesser than that (as the load is applied, pressure increases, the gas compresses, volume dectreases and hence to cover the same movement, more gas is required)
This are the basic differences, along with others eg
- pumps work with different clearances, obviously due to different viscosities.
- the relief valve lands up in some problem since small reliefs makes a large gas to escape, instead of the proportional escape in liquids.
- Safety norms are much more rigid, especially in high pressures, since a small leak creates a jet of the high velocity leakage, likely to be fatal (due to higher volume) etc.
A main use of these hydropneumatics we see in a lot of control systems (where the pressure is not high) and we use in some areas where the mechanical spring is replaced by the gas operated cylinders (with accumulators) and this gives a better spring characteristics unlike more rigid liquid operated cylinders (we need the compressibility for the kx value)
You can check the net for literatures - plenty are there.
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Hydraulic vs. Hydropneumatic 
Re: Hydraulic Vs Hydro-pneumatic
