If you keep having to add water/coolant there is a leak. There are really only three places it can go. Into the combustion chamber (head gasket), into the oil (again head gasket), or into the outside. If the oil doesn't look muddy (I know yours doesn't), It leaves it to a combustion chamber leak or outside. I know you pressure tested it last night but I've had small head gasket issues not show up until the engine is warm.
Outside leaks can be tricky, especially if the leak is small. As mentioned, dye works well for this. Some dyes will actually last through the combustion and if you have head issue it will show on the ground where your exhaust lets out and on the tip.
If the water/coolant is holding, you have a flow problem. Either air through the radiator, or water/coolant through the wet side. Water/Coolant flow can be a bad thermostat, water-pump, radiator, or hose issue. A full coolant flush might be in order to rule these out. Air flow is pretty simple to address. Just clean the fins. Sometimes the fan can go bad but you have already said yours is running so we know it's okay.
The last culprit that had also been addressed is the cap. The cap does two things (beyond sealing the radiator). 1- Maintain pressure in the system. For your Jeep this is 18PSI. This pressure raises the boiling point of the water/coolant. If the water/coolant isn't under pressure it will steam off at normal operating temps. 2- Allow for air to enter the system when cooling. If this doesn't work your hoses will collapse as the engine cools from operating temps. (This is the reason you replaced your cap to begin with.)
Did you have this issue before you replaced the cap?
