Not necessarily. Most good quality control valves have load checks that keep trapped oil in the loaded cylinders sealed off from the spools themselves. This is a one way (single acting) spool, so it may or may not have the same checks as the tilt spool, but it probably has something.
Both the pressure and return lines to this valve are hard plumbed steel lines right? So not much chance of any easy experimenting with interrupting the return flow and actually watching it while attempting to lift a load. I would be tempted to try and separate the return line somewhere, add a hose of some sort and return the flow straight into the tank filler port. Run the engine and attempt to lift the load. If the flow stops or drops to a trickle that would indicate the pressure and supply just aren't there. If it merely slow a little and continues, then that would indicate an internal bypass in the valve somehow.
Again, without a gauge in there somewhere you're still guessing at things.