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?

I would just replace the lifter

I went through and rebuilt all my lifters on my 440 on the Drivers bank after chasing a ticking sound (which ended up being the manifold warping)....All you need is a tub of oil 40wt...slip the lifter in, put an old push rod on the chuck and press it down (if you have a drill press...all the better)...use the tool to remove the keeper lock. take it apart...clean it....and then reverse the process....pump it a few times when you are done and slip it back in....since you go the cover off and don't have anything to do....do it to all of em...the others arent usually too far behind

I think I found the problem. By the dished out bottom end of this lifter, I would guess that the last rebuilder did not replace the hydraulic lifters with a new set. I will take your advice and replace them all.

Unless that hole is supposed to be there, I think I found my problem. Judging from the dished-out bottom end of this lifter, the engine rebuilder never replaced the originals!
I will take your advice and replace all of them.

I'd replace lifters and cam. proper breakin procedure (want it to fire imediately, no less than 2500 rpm & goose it repeatedly during the 30 minute procedure)/good high zinc breakin oil/black moly paste on lifter bottoms/cam lobes and ahead of time mockup your new lifters in bores cleaned w brake kleen & lightly lubed w WD40 & lift each lifter up off of it's lobe w your finger & let it go & it should fall back onto the lobe w a resounding "thunk" which tells you the bore/lifter clearance is adequate so the lifters will rotate which is critical. If not hone them.

There are lifters made with holes in bottom, but if the bottom is dished that shows the cam is toast.

OT those lifters w the mini hole can you drill them w a good hardened bit or does it require an EDM? machine? sounds like a simple/EXCELLENT way to keep the lobes lubed & there's a pressurized source right there, wonder why it was never thought of b 4