I had a similar issue with my 95 Ranger, it was leaking drip by drip out of the little lever that is normally connected to a vacuum line on the left side (when looking under the hood)
. My original thread from years ago is below, but what I ended up doing is going to an auto parts store (if it's the same part)
and got one, kept the receipt and if it started again I was usually able to take it back.. it's only a few bucks, but the truck was totaled in an accident (not involving me)
before I had a chance to check the system's pressure to see if that was the cause of it..
ongoing heater valve leaking problem