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Assuming it is a hydraulic pump driven by the PTO, you engage the PTO and pull a knob (like that big one on the dash to the Left of the steering wheel) There should be another one similar to that to engage the PTO. (or that one might engage the PTO and another one controls the dump)
You need to push the clutch in before engaging the PTO. You might want to look underneath the truck and see what engages the PTO, following the cable up to where it goes through the firewall.
PTO should be on the driver side of the transmission (assuming the trans is an SM420)
