Anyone who followed the saga of my efforts to transform a Russian Prim Compact from a stove with no simmering capability into a functional ‘user’ could be forgiven for thinking I’ve become obsessed by it! The existing pump wasn’t a model of efficiency. An O-ring serving instead of a leather pump cup rarely is. Made worse on this one by an over-sized bearing clearance in the pump cap. That’s not wear, it’s the original specification. Coupled with the short stroke, the aluminium flanges enclosing the O-ring could be felt to intermittently scrape along the pump tube wall in action. Eased by lubricant, it nevertheless gave the pump action a certain harshness. I’ve replaced it with this, made up from ‘brassie’ stove parts. Crucially, the leather pump cup is mounted in the way best described here by @kerophile : Essential too, to get rid of that slack pump cap, replaced by a close-fitting end cap. Presently a tight push-fit on the part of the pump tube projecting from the fuel tank, I’ll be installing a Coleman-pattern spring clip retainer.