So I looked at the Hypro pump on Rainbow (Otter is ~1000 miles south right now) and it's a 5300 series w/ (I think) rubber cups. I've used various displacements (1.5-> 3 gpm) and both rubber and teflon cups. They have all worked reasonable well, but as noted elsewhere are susceptible to problems w/ debris in the check valves. Good fine intake strainers are a must. They should also be mounted with the output port facing upwards to help clear air.
I was at the Columbia River Museum in Astoria, Oregon last week and took some pictures of the pumps on an 75 hp Atlas Imperial diesel engine; I find this sort of pump easy to understand and repair while underway. A simple rocker arm driving a plunger pump with easily opened/rebuilt commercial check valves.
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