Hi , these non-return-valves (NRVs) can be a problem on old stoves. If they are just sticking they can sometimes be freed-up by filling the pump-chamber with alcohol or kerosene, re-inserting the pump piston, tightening its cap and applying pressure. I hold the tank and rest the pump head against a door post or piece of solid wood. I then lean on it until the pump goes down into the cylinder and the liquid is ejected into the tank. If however you are having a paraffin back-flow problem, then as you realise, you need to remove the NRV. There is a special key for this but as you have a workshop you can easily make one. The flats on the head of the NRV which you need to latch onto are typically just under 5mm wide. If you get a 9in to 12in length of mild steel bar, and cut a notch about 5.1mm wide, and 5mm deep. at the bottom end, put a hole and tommy-bar on the other end, you have a homemade key which should do the job. The key has to be a snug fit on the NRV head and you need to apply some downward pressure as you turn the key, to stop it rising off, and damaging the soft brass head of the NRV. It is a normal screw fitting so you turn anti-clockwise to unscrew the NRV. The whole stove has likely been sitting dry for many years, so an overnight soak of the NRV in releasing oil, before attempting the dismantling, is a good idea. There has been some recent correspondence on the CCS website, initiated by my good friend Exeter Yak, who manufactures beautiful replacement parts for these veteran brass stoves. He is now producing replacement springs and washer "buckets" for NRVs. The actual washer is made of nitrile rubber, which resists the effect of kerosene in use. If you have 2mm thick nitrile sheet, you can punch plenty of washers. Some people get a large diameter nitrile rubber "O" ring, of about 3-4mm circular section, and cut short lengths to make washers. Your NRV washer was likely made in the 1960's or earlier so it might be getting a bit hard and tired by now! The NRV valve opens at about 10psi pressure, and allows air to enter the tank.The pump has to overcome this 10psi, plus any existing pressure in the tank. At the start of pumping this is only the hydrostatic head of the kerosene in the tank. However, as you keep pumping the back-pressure becomes greater. The 10psi NRV pressure setting, at rest, is really there to stop kerosene flowing back into the pump cylinder. I should say that the NRV valves generally have a soft lead washer, which helps them seal onto the tank seating. This washer sometimes stays in place when you extract the NRV and you are not aware of its existence. It is good to have the washer as if the NRV has been overtightened to get a fluid-tight seal they can be the very devil to remove later, and you sometimes are driven to un-solder the whole pump assembly to get access from the back! Good Luck with your fettling. Regards, Kerophile.