Vacuum Diaphragm Leak Check
WARNING: This page does not describe the selected car, but rather 124 other vehicles, including the 1991 Chevrolet Pickup, 1991 Chevrolet Cab & Chassis, 1988 GMC Vandura Special, 1988 GMC Vandura, and 1988 GMC Value Van. However, it is still accessible from the selected car via links, so may be relevant.
Insert pipe cleaner into vacuum connector pipe as far as possible and check for transmission fluid. If fluid is found, replace modulator. Gasoline or water vapor may settle in vacuum side of modulator. If this is found without presence of fluid, modulator should not be changed. If vehicle is to be exposed to temperatures 10°F or below, modulator MUST be replaced.