I'm a stickler for using a recovery tank. It's too easy to have voids and pockets with the plumbing of home-brewed coolant systems. Any air pockets cause the hot coolant to never become properly pressurized, which causes the "pocket" to boil and turn to steam, which can dump more of the coolant. After a fresh coolant change, I like to monitor the first half dozen warm-up/cool-down cycles by checking the recovery tank levels to see if the hot and cold levels are hitting the marks in the tank. If the level drops in the tank when it cools down, That tells me that the fill-up was incomplete and to add more fluid to the correct level (cool) in the tank. It may take a few warm-ups and top-offs for the system to stabilize. If the levels are erratic that's your clue to keep looking for the cause of the problem. Your comment about your head bolts and new gasket does sound like they could be under torqued....... but you didn't mention any leaking or coolant loss or percolating noises. (?) So I'm just throwing out something else that wasn't mentioned yet. I wish you good luck and a fast fix.