High coolant reservoir pressure

Hello, I need some help diagnosing a high resevoir pressure situation. 2015 cascadia dd13, 700k miles, pulling trailer I get the resevoir discharge hose spitting out coolant, only on hard pull or loaded trailer, never idling, replaced thermostat, flushed radiator, eliminated egr cooler as problem, eliminated air compressor as problem, did block test with blue liquid, no change in color even after half hour or bubbling, did bottle test start generating bubbles after thermostat opens at 170 degrees, no overheat condition highest its gotten is 225 on the gauge, no white smoke, clean oil, what else can I check to rule out head or head gasket, no fuel in coolant so don't think it's injector cups either...I'm at a loss, don't want to start removing the head only to find out nothing wrong, for nothing. Bought deleted, has egr pipe plugged at intake and also plugged from exhaust manifold to egr cooler on both connections, so no pressure at all at egr cooler and it's bone dry inside, tested fan clutch operation seems fine....any help would be greatly appreciated.

 

Yes, forgot to mention, replaced resovoir, whole thing brand new

 

I loose coolant thru discharge hose when pressure exceeds the 15 psi cap, no other leaks,

 

Great video, thanks