Coolant hose is not a "coolant" hose...

[Waterloo]

Still getting air in my cooling system! I’m up at TA having the air dryer installed and the front brakes. Drove here, cracked the hood only to see the coolant jug was wanting to push coolant out of the cap. I cracked the crossover petcock valve and released air until coolant came out. Took about 3 seconds. Can’t figure this out?

Also, prior to replacing the air compressor I did do a block exhaust gas test following the Quickserve instructions. I used fresh test fluid. No gas, passed test.