Coolant Reservoir pressure

[mrscottg89]

A week ago I had an issue where my cooling system blew up on me.

https://www.matthewsvolvosite.com/forums ... =1&t=45089
Since then I have replaced the heater core and have had the coolant light come on several times I have refilled about 5 times since heater core replacement and 10-15 since the incident last week. I still cannot find any evident spot it is spilling from. 4 days ago I drove it for a good amount and when stopping steam was pouring from the hood and the reservoir was empty when steam was finished. last night coolant light came on followed by loss of heat and oil light came on. I put stop leak in the radiator about an hour ago and topped it off drove for about 20-30 minutes and the coolant light came on and off and i lost heat only on the left side. I shut the car off and steam began flowing from under the hood so I popped hood and started car again and there was fluid all the way to the top of reservoir, but when i shut it off it all depleted into the reservoir not to be found again not under/in car no where? What could this be? Gaskets?