If you are experiencing a vacuum leak, your windshield wiper door vacuum switch may be the culprit. Modern Gen Auto has the replacement you are looking for.
This C3 Corvette 1968-1972 Windshield Wiper Arm Control Valve is manufactured to the OE specifications to allow an easy, direct fit for installation. This windshield wiper arm control valve is located under the passenger side wiper arm at the base.
The purpose of your Corvette's windshield wiper arm control valve is to open and close the wiper arm door, keeping your wipers shielded when not in use.