Who knows air over hydraulic stuff?

I have been looking at pneumatic swing clamps for a particular job. And even used ones in the size range I need for proper clamping pressure and really spendy. So I thought maybe hydraulic swing clamps would work. Besides being much less expensive the clamping pressures are much higher. The high pressure could be a problem. Looking at air over hydraulic pumps makes it obvious that this could be the much less expensive and more compact route. But I have questions. All the pumps I have been looking at are for single acting cylinders. Can the output from one of these types of pumps be run into another valve so that double acting cylinders can be used? Like the way the valving works on my backhoe for example? And how is the pressure varied? By how far the valve on the pump is opened? Years ago I operated an Enerpac double acting air over pump on a hydraulic press. The pump had a direction lever and a pressure lever. The pressure lever could be pressed very lightly so that only a small amount of pressure made it to the ram cylinder. But all the pumps I am looking at just deliver pressure and then release it back into the tank. I assume but don't know that the valve must have a pressure holding position. A friend just reminded me that I already have an air over hydraulic pump. The 4th (rotary) axis in the Fadal mill connects to this pump for clamping, so that the table doesn't rotate from cutting pressure. I'll need to see what the capacity, fluid wise, is and how much pressure it can deliver. and whether it is adjustable. If the thing will deliver enough fluid fast enough at high enough pressure then I can use it. But what if the pressure is too high and the unit doesn't have an adjustment? I should just be able to add my own pressure regulator, right? Thanks, Eric