I finally solved a mystery (for me at least!) of what was going on inside the mini-pump cap that allowed it to act as both a non-return valve (NRV) and safety relief valve (SRV) at the same time. In case you were wondering here is, from left to right; the mini-pump, the mini-pump cap and a plain cap for the 8R stove shown in this picture. My interest was more than academic as the two mini-pump caps on both of my 8Rs are leaking and I would like at least one working to avoid having to shuttle my one good plain cap between the two stoves. The cap has an NRV that is like the NRV on most stoves - a rubber pip sitting on the end of a spring held in a cylinder that presses onto a small orifice in the cap. This is on the right in the above picture. In the pump cap that orifice runs through the middle of a little piston that is inserted from the top or outside of the cap. A rubber seal (like a square profiled O ring) fitted into the cap should stop leaking when the shoulder of the piston is pressed against it by a strong steel spring. This is held in the top of the cap by the hollow penta screw. The darker coloured steel spring and small piston are to the left of the cap body in the picture above. Looking from the outside (or the top of the cap) you can just see the seal in the bottom of the threaded hole below. If you flipped the cap over and looked from the inside you would see the hollow end of the piston right in the middle of the cap. In the image below, the piston is fitted. It provides a sealing seat for the NRV pip. And below it is sitting beside the cap so you can see where it lived when fitted. Here's the piston below. It's hollow so air can be pumped in and pass down the middle of the piston. Normal pressure can't escape as the NRV seals against the end of the piston. The stronger top spring holds the piston itself down securely against the square seal from the outside. If the pressure became too high inside the tank, gas would be forced out past the outside of the piston through the opening in the centre of the cap and it should lift the piston itself off of the rubber seal, providing the required pressure or safety relief feature. Genius!