post hole digger keeps turning.

My search won't turn anything up on this.
2001 Case 580 sm
It has a solenoid operated valve (I call it a hammer valve) that's inline before the main hoe valve bank. Made by Sun hydraulics. When you energize the solenoid The digger motor speeds up and is controllable with the flow control knob on the valve (remote cable operated). When you take power away from the solenoid the digger motor only slows, won't stop. The flow control doesn't seem to effect the speed when the power is off. Just slow. I get the same results when I use the manual button on the end of the cartridge where the solenoid coil is.
I figured it must be the cartridge that the solenoid coil is on. I took it out and it seems to do it's job and the o-rings look great. Used air to blow it out and blew air into the hole in the block it came out of. Put it back together and it still does exactly the same.
Anybody been here before? Got any idea where the problem might be in the valve block. Any ideas?

pretty sure I've found the answer. Apparently the solenoid valve by Sun that was used to do this job needs a certain amount of back pressure to shut off. So adding a 90psi check valve in the pressure line to the motor does the job of stopping the post hole digger when you want it to stop. Here's the link. http://burlingtoninstallation.com/pdf/may_techbrief.pdf