Well the plot thickens

With just the Enerpac flex hoses, sealed at their far ends by virtue of the male quick disconnect, I can pump either of them up to 9,000 psi, and they will release when the other direction is operated. You can tell that they release, as when at that pressure the hose is very stiff and tries to unwind.
With my test fittings made up this morning that allow me to fit 16,000 psi gauges on the ends via female quick disconnects that I have removed from the cylinder, exactly the same operation. Pump one up by pressing the button on the pendant, let the pendant button relax and the pressure is retained. Press the other pendant button, and
after a short delay the original side is relaxed and the new side pressures up. Now I notice that the original side doesn't relax until the other one builds up a bit of pressure hence the delay.
So I removed the robbed female fitting from one gauge and fitted it to the top of the cylinder - inspecting it's mounting hole carefully - I was sure it WAS just a hole and no valve was lurking in there

Now I can pressurise the cylinder such that it gets to my set point of 4,000 psi AND release the pressure, but remember the other hose is not connected and still blocked by it's q/d fitting.
Returning the other hose to the bottom of the cylinder I can no longer release the pressure.
My tentative conclusion at the moment is that in the 'UP' direction the hydraulic pressure isn't building up enough when connected to the cylinder to operate whatever it operates and I need to find out WHY.
...all very confusing, trouble is that a few years back when I rebuilt this press after it's disastrous fire, I don't think that the piston seals were correct - certainly it never would get to the pressure that it should have and I suspect was leaking across the piston - this may have been overcoming the issue that I am now seeing
