Isx15 popping radiator caps

[FnG92]

The line going into the thermostat sprays/pushes out coolant and the side going to the radiator fins sucked the cup empty. It's a super short hose so I cut a cup to get into the small space. I did cover my finger over the side going to radiator and it didn't feel like it was building pressure. I'm going to get another cap right now and fill my coolant back up and try the little black hose again.