The vent flaps were made of fiberboard, or something like it, and disintegrate with time. Mine were in very bad shape when I bought the car; you may have the same problem. You will have a lot of heat coming in the cowling if you don't have the seal at the back of the hood and your vent flaps are not closing or are in very bad shape. (If you have access to the AOAI magazine, issue #170, page 49, I wrote up in fair detail what I did to fix the flaps; Reader's Digest version follows.)
To repair a flap (I'll speak singularly) remove the cover and disengage the cable. Push down on the bottom where the pivot rod enters the 'mount' and push up on the pivot rod itself, then pull the bottom of the pivot rod toward you; this removes the flap valve. I made a template from paperboard and then cut the final flap from thick gauge aluminum. Attached the flap to the pivot rod with pop rivets. Reverse to reinstall. Having the front seats out is a must.
If yours acts like the vents are not opening or closing, the screws which hold the cables in place under the control panel may be lose letting the cables slide back and forth with the action of the levers.