If something is not working at all, it seldom makes a difference if you have precisely 853 psi of pressure when 856 is specified. It's far more likely that you are actually getting nothing, when you ought to be getting something. You don't need precision instruments.
If there's a hose leading to the hydraulic cylinder in question, often you can tell if "there's pressure" just by watching and feeling while someone shifts the directional valve.
The solenoids on hydraulic directional valves generally have pilot lights on them so you can tell if "there's power" just by watching while someone activates the controls.
If you have pressure but not movement, you need to focus on the mechanics of the clamp.
If you have power but no pressure, you need to focus on the hydraulics at the valve bank.
You may need to trace the path of hydraulic hoses and pipes through the machine to identify with your own eyes exactly which valve is actually connected to the clamp. So be it.
Disassembling the entire machine prior to basic diagnostics is seldom the best course of action.