I'm trying to figure out how to create a seal for a very low pressure gravity feed system. Essentially its a low pressure solenoid valve. Is there any equations to estimate the amount of force you would need on a o-ring to seal it from any leaks of pressure $p$ its placed in a spring solenoid combination so I need to have this as low pressure as possible. Any ideas on what force I would need from the spring to seal a liquid the pressure is due to a height of about 40mm with a liquid like water. So if my math is right that's $$ \rho g h = 997 kg/m^3 * 0.04m * 9.81m/s^2 = 391.22\ pascals $$ at the o-ring so pretty small amount of pressure.
Also what material would you suggest for the O-ring I have considered, I have used 50 Durometer with no success and am thinking about moving to a silicone based O-Ring. I can't increase the force of the spring because a solenoid will need to overcome it and i'm limited on space.