The real pressure seal on the piston is the rear "O" ring, although the cup seal behind it can also hold pressure as I had a gun with the "O" ring damaged and the gun still held air. The front "O" ring on the piston is more of a guide and it is that which you can feel hitting the uneven surface where the rust has stuck which transferred from the piston's corroded rear metal section. Some pistons don't have two "O" rings, they just have the rear one along with the cup seal. If in doubt replace the piston as once you get rust on an item it is more susceptible to rusting there again due to the rough surface where rust once was. Also guns need to be thoroughly rinsed after use, especially vacuum barrel models due to the water trapping mentioned before and that includes pushing the piston back with the then exposed surface being flooded with freshwater and then being let out. A gadget can be made to replace the spear in pushing the spear back a short distance, something like this one from Nemrod, however it needs to be of the shaft diameter or slightly less where it penetrates the muzzle.