Sounds like you've got the same thing as I did. The vacuum accessory hose that goes to the vacuum ball is probably dryrotted away. The hose itself runs from the passenger side of the plenum to a check valve and then to a T fitting. From there the hose routes to the vacuum ball (on the driver's side, just forward of the fenderwell) and the firewall, going to your vac-elec solenoids (under the glovebox).
The engine creates vacuum in the entire line, including the ball. When the engine is under load or high RPM, the vacuum drops in the intake and the check valve closes due to higher vacuum in the ball than in the plenum. This allows the vacuum to remain to keep whatever mode switch active until engine vacuum returns.
In my case, the hose was rotted away at the ball itself, a quick PepBoys vac hose replacement fixed it, however you'll want to check all along the lines. It was originally thin plastic line, which in a 1995 vehicle is almost definitely cracked and brittle by now. Its not a difficult fix, and takes perhaps fifteen or twenty minutes (give it half an hour or so for first timers).