That sealed distributor cap will cause issues without a vacuum line pulling out the condensation. The previous owner probably had issues in water and rain (have a can of WD40 handy, and the allen wrench for the screws), so he used silicone to seal. Problem is that the atmosphere itself will cause condensation in the cap, and without a slight vacuum, it will build up on humid days.
Also keep in mind that its a slight vacuum. not full vacuum. Full vacuum will race the engine as a vacuum leak. Keep the cap and vent holes sealed, and It will pull air from the distributor shaft, coating the cap and contacts with a nice film of oil. been there. You may want to take the silicone off. At least make sure the silicone isnt closing the vent holes on the distributor base.
Most of our trucks no longer have the little right angle restriction valve on the distributor, and i'm not sure if they are available. I simply placed a small ball valve i had laying around into the vacuum line. Close it when you first install. Start the engine. Get the engine running right and nice and warm... Open the ball valve slightly until the rpms increase, then back it down just a little. It should be close to the proper vacuum. If not, open it just a tad more. It will take some practice, but you will find the right amount of vacuum to keep condensation out without causing a vacuum leak.
Engine troubles? Try here.