If the schrader can flow air faster than the leak port, you'll be fine. If it's loose enough that it won't seal like that, it will probably slide under its own weight. Just tip it down as you fill it. If it still won't seat, use a small spring.
Once the pilot is opened, air typically does flow back through, but because the pilot is much larger than the equalization hole, the air leaves faster than it can come in, and the pressure drops. If the equalization hole is too big, the gun won't fire. This is actually a common problem for homemade piston or diaphragm valves.