I have had a similar problem with my truck (83 D 250, 318, EQ Magnum head conversion, Melling "stock replacement" 360 cam, (compare to COMP 252) Summit 7.650 pushrods; 1 lifter from either cyl # 2 or 4; I just went and pulled the intake and looked; cam looks fine, chucked an old pushrod, left over from the "stock" days of this engine into my drill press; on #4 Intake (3rd valve back, from the front of the head) it was like that "squishy spring" mentioned way back in the thread; I had gotten my lifters from the local Car Quest when I did the swap; they'd sold me a set of lifters whose brand name I did not recognize; i was hesitant on using them for that reason; but I did....
the rest, I bent pushrods in the drill press (NO I did not have the DP running) and on the 2nd PR, I got a LITTLE compression of the lifter with oil weeping out the hole in the side; this one, while not exactly "easy", was the only one that readily went down, and the only one I was able to get 1/4-3/8" of compression out of.... and I did not bend a pushrod in the process! My truck has 15K on the conversion to mag heads (2K of that , just in the last 2 weeks) and that "1 lifter" has been gettin steadily worse; I noticed it faintly almost from Day#1 of the mag head swap. (why I took out the MP conversion 7.625 pushrods and ordered the Summit 7.650's, on account of "that one lifter"... so now I'm off to grab the phone and see who has LA 318 lifters on the shelf; how many I swap out, I guess will depend on who has what IN STOCK; I need my truck back going; if I can find someone that has even 2 in stock (I'll replace both on #4 at a minimum) I'll be happy. I know I should do them all; but with money as it is these days, you all know how that goes;