I am confused about the reservoir being part of the pressurized system. Is it a reservoir or a recovery tank? If it is for recovery it shouldn’t be under pressure and would expalin a loose fitting cap.
Some cars carry the reserve coolant within the radiator - this is the case with most older cars. Most newer cars have a very compact, fitted radiator that’s not readily accessible for service, and use a pressurized reserve tank that’s easier to reach, fill and check the level on. These are not the same as overflow tanks, which are typically not pressurized.
Well huge puddle of coolant today and so I crawled underneath and yep radiator is shot. Good news is it looks like I can swap it out myself so new radiator hoses and since I have the water pump and thermostat already I’ll just swap out the whole system.
Replaced the radiator (huge leak), waterpump (corrosion where there shouldn’t be any), thermostat and upper and lower hose. No leaks!! 
$500 
for everything but new cooling system I guess is worth it.
One last question. The fan turns on when A/C is turned on but it doesn’t run when parked/idling at temp. I know these are all computer controlled so maybe the initalizing conditions are not met but is there a way to test this?
It should come on whenever the temperature at the appropriate sensor or switch reaches a certain point, typically around 200-210°F. It can take a while to get there just idling. The temperature can be monitored with a scan tool or with an infrared thermometer.
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It’s always a Jeep.
Seconded. No hot coolant to heater core and weirdly fluctuating coolant temps? New thermostat, stat.
ETA: Wherps, just noticed you’d already rectified the problem.
That’s what I’m worried about. It’s not kicking on when at temp (I noticed while burping it) which AFAIK is a relay or burned out motor. It does kick on with AC so looks like I need to do some more research.
2WD or 4WD?
Which engine?
The fan senses the a/c head pressure and turns it on responding to freon pressures. Make sure you have the a/c off while checking your cooling fan or it may never get up to temp. If you have a 205 thermostat the fan might be as much as 10 degrees hotter.
The radiator cap works differently on these systems than it did on the old, unpressurized-overflow systems.
On the old radiator caps, the spring was the pressure valve that allowed water into the overflow.
On the new readiator caps, there is no pressure valve in the cap. You don’t load the spring in the pressure cap by closing the cap. The cap just locks on.
When a car has one fan… The fans on most cars I have worked on only come on when it is a hot day and you leave the car idling for about 10 minutes. They will not come on at all when it is cool.
When the car has two fans… One fan is for radiator cooling and the other fan comes on when the A/C is on and may only come on if the A/C needs it. With some cars, the second fan always comes on when the A/C is turned on.
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If the car is not overheating when idling, then no problem with the fan not coming on.
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To find out how the fan should work, I would get the factory service manual set of books or DVD which would include the electrical wiring diagrams. Then look to see how the fan works. There many times is a theory of operation blurb for each major part - explains how it should work. Like: When engine reaches such and so temperature, fan is activated. Or if A/C turned on. Or if A/C reaches a certain pressure.
Or you can look at the wiring diagram and it will say in small print what temperature a sensor will switch on at.
You may get lucky finding this information for your specific vehicle on the internet, but there is a LOT of misinformation out there, especially about electrical things.