It is either a seal issue with the rings or valves, or a mechanical issue with the valve action (bent pushrod, loose/broken rocker, defective lifter, worn cam lobe(s).) Try a squirt of oil in the cylinder to rule out ring seal, and inspect the valve train components for that cylinder. See if the valves depress the same as the other cylinder’s valves. A leak down test is the next step to determine the source of a leak.
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