Coolant spraying... What is going on?

[Waterloo]

Ok, did the overhead, installed new CAC, put everything back together, went to start and I have coolant spraying everywhere! Coming out of the coolant bottle, and somewhere else. I shut it down, thinking WTF! Now what!

When I took the coolant bottle off to replace the CAC, I drained about three gallons of coolant out of the system to drain the bottle. I filled it up after getting everything back together and burped the motor.

I looked all over the motor, pressure tested the cooling system, nada.

Restarted the motor, purring like a kitten, the coolant is keeping its level in the bottle. Sits overnight, and pressure has built up in the system near as I can tell. Could it be something as simple as a bad radiator cap?

Now I start it today, and same thing! Spraying coolant everywhere, after sitting overnight. Restarted and all is fine...

Don't tell me a head gasket.