Hi there,
I have had a case like Sean describes a few times, where one phase was (partially) interrupted at the fuse, contactor and motor OL trip. In such a case, the easiest way to spot the problem is to measure phase voltages under load, from the motor terminals upward, towards the supply.
Almost all hydraulic pumps are constant flow rate pumps (positive displacement). If you pinch the output, ex. by operating a valve, the pump will try to maintain it's flow rate and as a result, pressure will increase. If the pump has to work harder, it will need more power and as a result current will increase. There are a few things you can check without having to decouple the pump from the motor.
1. As suggested before, decouple the pressure side at the valve block and put the hose in the reservoir. This will effectively have the pump run at free load. Check motor current but also check whether the oil is clear of bubbles. If the oil is foamy, that meant the pump is cavitating, not good. If this is the case, check the filter in the reservoir. If the oil is milky white, there is water in the reservoir, very bad.
If the pump works normally and the oil is OK, the hydraulic system may be dirty or the valve does not open up correctly.
Reconnect the pressure hose to the valve block. If the machine is in the unclamped position, there should be almost a much oil coming out of the return hose, as coming from the disconnected pressure hose before.
2. Check whether the correct viscosity oil has been used. A lower viscosity means the pump has to work harder to push the oil through the system, especially at low temperatures.
Good luck.