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?

The pressure reducer/relief is also pilot operated and the drain from the valve is also connected to DR2. Its always good to run pilot drains seperate from the main return line.

The GA of the power unit and the schematic only show DR2, which is the Y port drain coming from one side of the manifold. It then connects to the power reservoir via a hose.

You say you replaced the valves and the system worked better. That would rule out the drain line as being the problem.

I think the original PO check had a fault...simple as that.

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