As the poet said, "Pump won't work 'cause the vandals took the handle." This may be of some use to others grappling with the hopeless SHURflo user manual troubleshooting page [translated from
what?!], FWC's troubleshooting PDF [useful, but no cigar for my problem], and the Wild West of Youtube.
Filled my tank, turned on the pump, pump ran continuously, but no flow. After much head scratching, checking intake and output lines [all clear], and puzzling over the aforementioned, I finally came across this video:
BINGO! Basically, there is a spring-loaded "check valve" [mentioned in one of the earlier replies to this] inside the plastic manifold on top of the pump. It opens and closes to alternate intake and output in the cycle. It can get stuck. When stuck, the pump runs, but neither draws water nor pushes it. When I took the manifold off the top of my pump, mine was stuck. All I had to do was push on the valve, and it freed and returned to its normal spring-loaded in/out operation. It seems it can get stuck from: 1) debris jam, 2) calcification [mineralized water drying up in the pump over time, especially sitting dry all winter], or 3) just plain crankiness [my problem?].
FWC really out to add this to their "Water Pump & Water System" PDF--a
lot cheaper than replacing a perfectly good pump.
Of course you've got to pull the dang thing out and put it back into the hole they built around it, but thanks Mom and Dad, for giving me skinny arms!