dual pump high-low unloading valve help please

[timmer62]

I have a small power pack that I use in my concrete boom pump business. The high flow, low pressure circuit dumps to tank at 44 bar and the low flow, high pressure circuit dumps to tank at 240 bar. When I fire up the unit and dead-head it, the pressure first goes to 240 bar, then oscillates a few times between 240 bar and 44 bar, and then it settles in at 44 bar.

I can raise the pressure setting on the high flow, low pressure circuit to around 100 bar before it stalls the honda engine, though this makes the unit barely useful. This tells me that the low pressure circuit is healthy.

I thought it might be the check valve, but I have never seen them oscillate. Don't understand how the pressure relief valve would fail in an oscillatory manner either. I am hoping the forum can help me figure out what might be going on.

The hydraulic schematic for the valve is attached. Any ideas what would cause this behavior?

 

[Stick.]

I marked this to be moved to the fluid power forum as it's more appropriate there.

Can you post a schematic for the system? It would be helpful to know the other components in the system. Also, which relief in your schematic is high pressure and which is low pressure?