If the tires are like my SM F-86 there is a machined alum spacer inside the tire where each rim half attaches to. Mine has a machined groove in the spacer with an oring set in it. When you fill air thru the brass nipple the oring expands out of the groove and then reseats when you stop pumping air in which acts like a one way check valve. I am not a big fan of this system and with SM would go to a solid tire like all the other mfg`s. The tire is not very beefy and will wear down quickly.
V..