My 71 has 3,000 miles on it since the engine was rebuilt. I noticed that the intake pushrod on the No. 1 cylinder had about 1/4" of play between it and the adjuster stud on the rocker arm. Upon further inspection, the hydraulic lifter was "squishy" and would not resist down force on the pushrod. The lifter was not filled with oil and was only pressing on the pushrod by the spring inside the lifter.

The No. 1 intake runner at the head has a dark brown, carbon-like buildup on it. None of the other intakes has this crud. I expect there may have been a problem with the hydraulic lifter for No. 1 intake for some time.

Is my lifter bad or is it not getting oil? Is there a way to diagnose, or should I just replace the single hydraulic lifter? What's the best way to fix this?