I have 5 month old Royal Enfield 350cc with only 4600 km. My bike stopped in the middle of the highway ... fuel was leaking ... I went to the service center they told me there is an issues with fuel tank ... it's leaking ... I got the fuel tank changed ... now I am facing air pressure issue with my new fuel tank ... I again visited the service center ... they checked the vacuum line & changed the valve ... but still I am facing the issue ... my bike gets stopped again and there is air pressure in my fuel tank ... please suggest what to do ... I have visited the service center many times but issue is still reproducible.
The combination of "leaking fuel" and finding pressure or vacuum in the fuel tank indicates that something is awry in the fuel delivery system (petcock, fuel line, carburetor, fuel tank) and/or in whatever system the bike uses to manage venting from the fuel tank (fuel cap, gasket, included vacuum port or valve, connecting vacuum hoses, something else).
Both of the symptoms you report are abnormal, and neither should occur. I don't know how these systems are designed on this model bike, however, so I can't give you specific troubleshooting ideas. You must find a competent mechanic to examine the machine, and find and rectify the faults.
If the bike is still under warranty, return it to the dealer.