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A fuel tank venting system includes a fuel cap, a vent diaphragm and a vent disc. A valve cavity with a substantially concave bottom is formed in a bottom of the fuel cap. The vent disc preferably includes a disc flange and a diaphragm projection. At least one air passage is formed through the vent

A fuel tank venting system includes a fuel cap, a vent diaphragm and a vent disc. A valve cavity with a substantially concave bottom is formed in a bottom of the fuel cap. The vent disc preferably includes a disc flange and a diaphragm projection. At least one air passage is formed through the vent disc. A substantially convex surface is formed on a top of the diaphragm projection. The vent diaphragm is retained between the substantially concave bottom and the substantially convex surface. When air is needed for fuel flow, a vacuum within the fuel tank pulls the perimeter of the vent diaphragm downward, which breaks a seal with the substantially concave bottom. Air flows around the unsealed perimeter of the vent diaphragm through the at least one air passage. In a second embodiment, the fuel tank venting system is contained in a wall of the fuel tank.

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1. A method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere, comprising the steps of:providing a fuel cap having a valve cavity in a bottom thereof, forming at least one flow passage through said fuel cap to said valve cavity; inserting a diaphragm into said va

1. A method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere, comprising the steps of:providing a fuel cap having a valve cavity in a bottom thereof, forming at least one flow passage through said fuel cap to said valve cavity; inserting a diaphragm into said valve cavity; and inserting a vent disc into a bottom of said fuel cap, said vent disc including a diaphragm projection, forming at least one air passage through said vent disc, a moveable outer perimeter of said diaphragm sealing a bottom of said valve cavity, a vacuum in the fuel tank unsealing said diaphragm to provide air flow to the fuel tank through said at least one flow passage and said at least one air passage. 2. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 1, further comprising the step of:providing said valve cavity with a substantially concave bottom. 3. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 1, further comprising the step of:forming a substantially convex surface on a top of said diaphragm projection. 4. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 1, further comprising the step of:said diaphragm being fabricated from a flexible material having memory. 5. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 1, further comprising the step of:forming a filter cavity in a top of said fuel cap, inserting an air filter into said filter cavity. 6. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 1, further comprising the step of:forming a threaded bore in a bottom of said fuel cap to threadably receive a fuel tank fill spout of a fuel tank. 7. A method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere, comprising the steps of:providing a fuel cap having a valve cavity in a bottom thereof, said valve cavity having a substantially concave bottom, forming a flow passage through a top of said fuel cap to said valve cavity; inserting a diaphragm into said valve cavity, said diaphragm being fabricated from a flexible material having memory, a moveable outer perimeter of said diaphragm sealing said substantially concave bottom; and inserting a vent disc into said valve cavity, said vent disc including a diaphragm protection with a substantially convex surface on a top thereof, forming at least one air passage through said vent disc. 8. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 7, further comprising the step of:forming a filter cavity in a top of said fuel cap, inserting an air filter into said filter cavity. 9. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 7, further comprising the step of:forming a threaded bore in a bottom of said fuel cap to threadably receive a fuel tank fill spout of a fuel tank. 10. A method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere, comprising the steps of:providing a fuel cap having a valve cavity in a bottom thereof, said valve cavity having a substantially concave bottom, forming at least one flow passage through said fuel cap to said valve cavity; inserting a diaphragm into said valve cavity; and inserting a vent disc into a bottom of said fuel cap, said vent disc including a diaphragm projection, forming at least one air passage through said vent disc, a moveable outer perimeter of said diaphragm sealing a bottom of said valve cavity, a vacuum in the fuel tank unsealing said diaphragm to provide air flow to the fuel tank through said at least one flow passage and said at least one air passage. 11. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 10, further comprising the step of:forming a substantially convex surface on a top of said diaphragm projection. 12. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 10, further comprising the step of:said diaphragm being fabricated from a flexible material having memory. 13. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 10, further comprising the step of:forming a filter cavity in a top of said fuel cap, inserting an air filter into said filter cavity. 14. The method of venting air to a fuel tank without allowing fuel vapors to escape to the atmosphere of claim 10, further comprising the step of:forming a threaded bore in a bottom of said fuel cap to threadably receive a fuel tank fill spout of a fuel tank.