my 94 skylark has a vacume line going down to the vacume modulator and i checked the line out but at the end of the vacume line by the modulator there was a check valve that reduced the vacume,is that suppose to be there or is it suppose to be full vacume. the haynes manual i have just shows a line going down there kind of vauge the vacume diagram on the car doesn't show much either.

Last I knew it was a check valve. Should something happen to the modulator, you won't have a constant vacuum leak. Plus I think it also prevents trans fluid from coming up the line should it leak.