2012 with 279k miles overheating

I recently acquired this car cheaply and replaced the headgasket and cleaned the egr system.
Water pump is working heat currently is not and I'm getting an overheating issue pulling the mountain with antifreeze coming out of the relief on the coolant reservoir.
Fans- working
Water pump- working
Heat- not working
Egr- spotless
Egr valve- clean and functioning
techstream shows everything working as should be no faults other than B1503
Sometimes the reservoir has pressure other times it doesn't
No leaks with the headgasket or miss fires
Engine coolant isn't low
Previous owner used Red devil block sealant ‍and I'm leaning towards radiator
Opinions are welcome