1969 Corvette windshield wiper door problem

Can anyone help - I have a SB 1969 corvette and the wiper door does not stay closed when I turn the car on. The lights are fine so there is no vacuum problem there. It's just the wiper door which slowly
opens when car is started. Before I start replacing the wiper door actuator and the vacuum canister (although I believe this part should be good since the lights stay down fine) where should I look? If I get a vacuum tester, what should I start checking first - the hoses?
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I think I'd start looking at control valves, maybe one has a slow leak. You determined there's vacuum in the system, and since the door opens then its actuator and such are in working order. Those eliminated, the slow opening suggests a leaky valve to me.
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