Found out that if you start nmbd with a workgroup where you are the only member. Elections are not run immediately and nmbd waits 5 minutes before becoming the local master.
After analysis with Jeremy we found out that this is caused by a bogus assignment in check_master_browser_exists()
857c2e4407a0f4fcee721372ffed5366bc3051f9 fixed it
Thanks Jeremy, now it takes 15 seconds to become master browser from startup if no other machine exist in the same workgroup