On a Foster fitting with the sealing rod, think of the air pressure in the air cylinder as the return spring.
So, you have pumped 1000 psi into the rifle's air cylinder. When you exhaust the pressure at the Foster connection, the air inside the air cylinder tries to rush out to atmosphere. The rushing air applies force to the area of the check valve and moves it forward. Once the sealing surfaces mate, you have the full force of the trapped air pressing on the area of the sealing surface.
So, on an M-rod, you have about 1/4 inch OD since the guide pin is facing towards atmosphere.
This takes you to the formula of, F = (pi * Rsquared) * cylinder pressure, which leads you to the amount of force applied to the face of the check valve.