996 eng check light exactly at 3/4 and 1/4 fuel level.

[1969jboy]

2001 911 gets an eng check light like clockwork at 3/4 and 1/4 fuel tank levels with codes 1128 and 1130. Is it a problem with fuel tank vent valve, charcoal cannister?
Light never comes on at any other time. Runs great, starts great, and passes emissions test fine. Light never comes on if I only fill the tank less than 3/4 full. Thanks, hate to just take it to my mech without a clue.