so from what you are describing, when you add an external power source to the solenoids the functions return to working.

on this system on this yale truck that 'fluid power system board' as you put it is called a MIB, this is other wise known as a Main Interface Board.
This controls the functions and also is programmable to control how the functions work and unfortunatly is also what supplies the power to the solenoid coils when a function is selected to make them operate.
find the connector from the harness going to the mast control cable and check the voltage outputs at the connector, it may be labeled a PWM cable.
If you find the outputs are the same voltage as you are seeing at the coils on the other end then this is most likely your problem.
From what it sounds like the MIB has a bad output voltage or could be programmed incorrectly.