A few weeks ago I posted a question about the wiper door on my 1970 corvette- it worked great all spring but then suddenly would not close with the car running (new hoses installed last fall). I took your advice and used a vacuum tester and worked backward from the wiper door actuator vacuum cylinder. Everything worked including the wiper park safety switch. Looking under the dash I found a hose which was slightly deformed where it curved through the firewall- it had gone soft in the hot weather and this was enough to prevent vacuum supply. Thanks for your help, this is a great discussion board! Scott