HELP HELP!! tank pressure!!

[Sicmaro]

ok, so my vent hose is sparying gas out of the tank. i start driving and after a short time it starts coming out. if i leave the car running and get out and take the gas cap off it stops coming out of the vent. i understand it releasing the pressure. what i dont understand is why there is so much pressure that it is coming out the vent hose.

ricks hot rod gas tank with in=tank pump. using a vette filter/reg. and this is a 72 camaro with an ls6

any help would be great!!!!!!!

sorry if this is in the wrong forum.