Hi, I thought it might be useful to post some photos of a typical pump assembly from a classic brass tank stove: The pump is a clever design. The whole piston assembly is designed to "float" between the shaped radius at the end of the reduced section, and the small nut on the end of the pump shaft. On the push stroke the piston seats on the radius and this forms an air-tight seal. Air is pressurised and is forced through a non-return valve into the tank. On the pull stroke the piston assembly moves to the nut on the end of the reduced section, and air can now enter the cylinder past the radius section. On the majority of pumps, such as this one, the piston assembly is like a drilled bolt and nut, which traps the leather washer in the correct position, and slides on the reduced section of the shaft. One or two manufacturers used a simpler but adequate piston, where the leather "bucket" sits between two steel washers. This piston design relies on by-pass flow, past the periphery of the leather washer on the back-stroke. Best Regards, Kerophile.