MF 135 isolating selector valve leaking

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Tractor came with purchase of property. Works good, however when using the hydraulics, oil leaks out of what I assume is the isolating selector valve. I have included photos, and wondering if anyone has any ideas on this?

I am novice, but keen to learn. Have purchased MF Owners Manual and Shop Manual but neither cover this valve.

Greatly appreciated with whatever advice anyone can pass on.