Vacuum line diagram?

Your vehicle actually has two ports on the booster. We need to find out if you have another line coming from this. If not then I suspect this other port that you have is for that. The other possible cause is someone just tapped into the vacuum system with a port and left it open. So that means we just need to find a rubber cap that will fit and cap it off.

Here is a guide that will help with vacuum testing a vehicle plus a chart that shows possible vacuum issues in case you need this info:

https://youtu.be/4ktw1X4W_-k

Please run through this info and let me know if you have questions. Thanks

That port used to go to the vacuum-operated cruise control servo, but as just mentioned, it's capped off if the vehicle doesn't have cruise control. Also, Chrysler probably had a golly-zillion of those check valves in stock, so even when they went to throttle-by-wire systems and the cruise control was fully electronic, they still used that valve until their supply got used up

For my other comment of value, it was not unheard of in the '80s for those caps on unused ports to fall off on GM and some Ford vehicles repeatedly. It was a minor but recurring issue caused by a leaking vacuum modulator valve on the automatic transmission. Transmission fluid would sneak out very slowly, but it was enough to get sucked up into the vacuum system where it rotted the rubber caps until they were loose enough to fall off. In every one of the few I was involved with, the fluid leakage was so gradual, the rubber cap was the only symptom with slightly-delayed up-shifts. The transmissions still shifted okay and were only slightly low on fluid.