A little idea suddenly occurred to me whilst stripping the coffee machine’s brewhead to replace the gasket. Why not try to reduce the backflush that happens after a brew so that less gunk ends up refluxing into the brewhead?
It’s a simple idea. Rather than ending the shot by switching the pump right off and closing the solenoid valve (thereby opening the brewhead to the backflush pipe which is at atmospheric pressure), why not scale down the pump power until the flow is really low (and so the brewhead pressure is, too) and only then switch off power to pump and solenoid? That way, hopefully, the pressure will bleed off through the coffee puck, with the water flow always going out from the brewhead and no gunk will reflux.
I’m trialling algorithms for this out now. It’s easy to tell if it works. When switching the machine on, if I pump some water through and it comes out brown / yellow then the last shot refluxed. I think the latest mod will do it, but I’ve had enough caffeine for today, so will have to wait for tomorrow to see whether it works.