Mine did this for awhile. I have a 1g so it's probably different. In my system was this little blue check valve. It has a diaphram on the inside of it that was stuck closed and was not venting. I removed it, filled it with wd-40 and used a small tool to prod the diaphram until it was loose. Then I dumped wd-40, refilled, reworked. over and over until I was satisfied.
I then reinstalled and haven't had the problem since.
1990 Eagle Talon TSi AWD 5 speed.