There is a special tool, but even that will damage the seat if you don't get it just right. Try putting the plastic top back on without the spring and poppet and pull straight up gently with needle-nose pliers while moving it in a circle.
It sounds like the manifold is washed out. The o-ring or backup ring will hang up in the eroded groove. Look in the manifold where the o-ring should seat and see if there's a groove. There's lots of info and pictures here:
Odd to hear the Arimitsu service is better than Cat. Just curious...of the 20 Arimitsus how many were purchased? I don't understand why you feel it is odd that they have better service. Actually, Cat didn't place the bar very high. They seldom, if ever chime in here to help out. James (no...
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The backup ring goes on the bottom if the valve is sitting upright on a flat surface. I dip the assembled valve in oil before I install them to keep from cutting the o-ring.