I'm restoring an 86. I've bought several center vents and finally have one that isn't cracked and doesn't have broken or repaired screw lugs. In getting ready to install the part in my dash, I noticed what appears to be a characteristic failure. Gravity closes the vent flap. There is a piece of felt that originally provided enough resistance against the thumbwheel to hold the flap in the desired position. All of the center vents I have exhibit this problem. I'm not sure if I should try to repair the vent by attempting to put in new felt (seems close to impossible) or use an 87 vent (heresy) which has no flap (because the 87 has the flap built into the heater box). Any suggestions?