[FWB]

hard to tell from the pics what the can is even for....

what hoses go to it? if its just for the overflow from the cap take the hose off and troubleshoot the leak without it, if there is that much overflow going into it then why?
1- radiator cap is too low of pressure
2- its the right pressure and your blowing engine compression into the cooling system, but you said you pressure tested at 18 then why didn't pressure go into the tank then? or maybe it wasn't 18, or maybe there is more pressure at running than you realize.

3- you simply have the system overfull
4- clogged radiator, collapsed radiator hose on the pump side.....

drain some of the coolant pull the upper hose and put a new t-stat in it.

there is no real magic wand here, if it ran fine before than something has changed......