But the pressure is what's confusing me. Why wouldn't the pressure relieve itself through the expansion tank? I might be understanding the function of the expansion tank cap correctly.

 

That is not the functionality of the expansion tank. What is happening is that pocket of air is circulating in your cooling system and it is causing it to pressurize. I would suggest next time, lift the expansion tank above the radiator (when the coolant level is low), let it sit over night and when the truck is stone cold, take the cap off and top off the coolant. Do this till the coolant level does not decline any more.

 

UPDATE
6 hours into tearing things apart
Removed upper plenum
Drained coolant
Removed alternator
Removed/ set aside (still connected) AC compressor
Removed/ set aside (still connected) power steering & ACE pump
Removed pulleys
Removed coil packs
Removed intake
Removed all brackets for alternator, AC, pump
Removed SAI
Removed valley pan


Issues
Broke 3 of the upper plenum bolts
Broke breather tube on passenger side valve cover
Broke passenger side SAI trying to remove
I need to go buy triple square sockets for valve covers and exhaust maniford

 

Nice work!

I used a 5/16 12pt for the valve cover bolts
12pt 12mm universal for the exhaust bolts
5/8 6 pt universal for the head nuts.

 

UPDATE
I would say I am officially at 8 hours tearing down and I am 90% done
I went and bought a univeral kit at Home Depot for $49, it has 5mm - 16mm

REMOVED Valve covers
REMOVED Exhaust Manifold bolts

Found a SAI locally to replace the one I broke
PENDING drill out the (3) snapped studs on the intake manifold
PENDING replace breather on passenger side valve cover
PENDING remove heads

 

That engine is pretty clean, good job so far. Good tools also.

 

UPDATE
Removed drivers side head

ISSUE
Rounded one bolt on drivers side and perplexed how to get it out. Thinking of drilling the head of the bolt off, remove the head, and attempt to PB Blaster and vice grip the remaining bolt out. Thought??
Broke the nipple off the top of the radiator when removing head bolts...

 

Get a 6 point impact socket on that bolt head, it will come out.

 

tried 6 and 12 on impact gun. Would not budge got frustrated and ended up rounding head of bolt

 

Others have reported good results with a bolt like the one linked below.


Irwin Tools BOLT-GRIP Extractor Expansion Set, 5 Piece, 394002