All this valve talk reminds me of a time years ago when I was chasing the MLT on an MCE unit with a UC4...all new stuff. Maybe a year later after running fine, this MLT stuff starts to occur. After several adjustments here, there, and days going by before it would happen again, I finally found the L/S leveling adjustment screws loose with the dot on the stem to the S side of the line. DANG IT! No wonder it wouldn't hold an adjustment on LU. Scooted that dot back to the line, tightened 'er up and bingo! Sometimes ya just- get- bit.