93 Mark VIII cooling system issues

[98LSC32V]

I have a 93 Mark VIII, only 71 k miles on it. Noticed message center saying low coolant so I popped the hood, could barely see any in the overflow. I added some water to the overflow (I know you aren't supposed to add there but I didn't have any tools for the crossover). Well I started the motor and drove around the block, there was an air bubble and the coolant temp started fluctuating. I turned on the heat and when I got back, turned off the motor, the coolant I added had dissappeared from the overflow. No leaks that I can see, no coolant in the oil, water pump replaced around 10k miles ago. Any ideas?

 

[87TTOPGT420]

Well It went some where, my old 1993 sprung a minor leak at tank on rad but that was with 180,000 miles...

Pressure test system check for any leaks...replace thermostat fill cooling system using port, run and watch gauge verify no more air pockets...seen this take up to 30 min with a mark viii before then cap system test drive and watch, also verify cooling fan oPeration at same time as waiting out any possible air pockets should come on and cycle off Twice before calling system filled and ready for road test

Atleast this is how I like to verify a cooling system problem