I have a system i want to get some advice on. Large milling machine, hyd system = counterbalance circuit : 2 counterbalance cylinders, pilot operated check valve, pressure reducing valve and a nitrogen charged accumulator. 1 additional pressure line going out to 1 other area of machine. My experience with this system is when hydraulics are shut off pump pressure immediatly drops to zero and counterbalance gage does not drop, holding system pressure for that circuit at the check valve. What I have on this machine is pressure is maintained at the pump for a couple seconds after pump is shut off, then a noise like fluid rushing back reversing the pump. Hyd gage drops to zero. C/bal gage also drops to zero, then quickly return's to system pressure. Is a brand new machine, have replaced the pilot operated check valve and the pressure reducing valve. Initially with the original components the c/bal pressure was slow to creep back up to system pressure after shut down, now with new valves = quicker response to system pressure.

The two smallest holes that you can see in the block are the X and Y ports. X is the pilot pressure to open the valve, Y is the drain port.

You are correct...sorry I did not read the drawing correctly. There is a seperate drain line for the y port. It goes back to DR2 on the power unit. That would be the place I would look.

I am sure the problem is with the PO check not closing.

Is the schematic correct because it also shows the counterbalance pressure at 5-5.5 MPA which is 50-55 BAR. The pressure gauge in the picture only goes upto 16 BAR?

Having read Bud's comments and looking at the schematic again, I now see a problem with the system.

The pressure line through the PO check is tied to the pilot line. You can see on the schematic that just after the pump flow enters the manifold, it goes up to the pressure reducer, left to the Y axis conveyor and right to the PO check as the pilot to open the valve.

With the PO check being open and having equal pressure both sides and with a heavy bias toward being opening, what is there to stop the valve staying open?

With the PO check open and gravity acing on the cylinders and the accumulator emptying, the pump is being motored. If there is enough pressure to turn the pump and the electric motor, there is surely enough to open the PO check.

I have never seen a system where a setup like this operates safely.

I think the pilot line needs to be supplied from somewhere else...not from the main pressure line.

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