Replaced my radiator cap with a new one from the Dodge dealer. I drove the truck to work that night and home the next morning ( 16 miles total). Truck was not driven all day. I checked the radiator reservoir and cap and there was no pressure when I released the cap. I know the cap was put on correctly so that's not an issue. The truck did not over-heat when I drove it so I'm baffled. Apparently the pressure in the system bled off through the cap during the day when the truck was parked. Anyone have any thoughts? I put the old cap back on until I can get another one from the dealer.

 

If you are saying the pressure went away after cool down this is normal. It doesn't build pressure until it is hot, and it will dissipate the pressure upon cooling back down. Now if you had no pressure and it was still hot (with thermostat open), then you have a problem. A pressure test can confirm (most kits come with what is needed to pressure test the sytem and the cap and all radiator shops or good shops should have one).