Last weekend there was a little time to do some upgrades and re-install the pump. It's now (a little) easier to pedal, the pump is more securely mounted, and the belt has stopped slipping.
I am a little concerned about the pump we chose. It's still very hard to turn, especially when it has water in it. I am hoping that it will loosen up with use. If not, everything else is good, so it will just be a matter of finding a better pump and swapping it out.
The bigger sprocket helped a lot. It's one that's normally sold to use with those nasty little gas motors that people mount to bicycles. (It was in our bike boneyard.) It made it necessary to lengthen the chain. Our first attempt at brackets allowed too much movement, so we swapped those for some flat right-angle brackets and stud hangers - much sturdier.
And then there was the slipping belt to fix. We needed to make the bicycle rim into a better pulley to cradle and grip the drive belt. After a few other botched attempts, we came up with the idea to put an inverted belt into the channel to make a place for the original belt. Yep, one of those underwhelming MH eureka moments happened.
Well, that's it for now. We'll see if the pump loosens up. Right now it pumps a gallon for every five seconds of pedaling, but a minute will wear a pedaler out. Still, we are getting closer!
May your garden love water you harvested yourself and may all of your pedaling be fruitful.