Tire deflation mechanism

Claim 1

In association with a road vehicle having a plurality of wheels equipped with pneumatic tires, each tire having a hollow stem containing a depressible check valve therein for tire-inflation purposes: the improvement comprising a deflator mountable on a stem of each tire on the vehicle, the deflators being collectively used to lower the tire pressures to predetermined low values when it is desired to operate the vehicle on soft terrain; each deflator comprising a hollow body mountable on the associated stem; internal wall structure within the hollow body defining a cylindrical chamber (32); differential area plunger means (24) within the hollow body located to align with the aforementioned check valve when the hollow body is in its mounted position; said plunger means comprising a piston (28) slidably disposed in said cylindrical chamber, a manually depressible pin (30) extending from one face of the piston outwardly through the hollow body, and a rod (26) extending from the other face of the piston through a wall of the cylindrical chamber and toward the aforementioned check valve: the cross sectional area of the piston being greater than the cross sectional area of the rod; spring means within the cylindrical chamber acting to move the plunger means away from the check valve; and a plurality of air passages (36) within the hollow body operable to transmit pressurized air from the check valve to the cylindrical chamber and thence to said one face of the piston; said air passages being spaced radially outward from the cylindrical chamber without going through the plunger means, whereby said one face of the piston is undiminished in area by reason of the passages; said one face of the piston having sufficient area that when the air pressure has a value higher than the predetermined low value the air pressure forces on said one piston face are effective to bias the plunger means to a position wherein the rod holds the check valve open; said spring means being sized so that when the air pressure drops to a predetermined low value the spring means is effective to substantially balance the air pressure forces on the plunger means, whereby the plunger means then offers minimal resistance to movement of the check valve to its closed position.