first, given coolant mods are fairly common, but their implimentation vary grately,
does your expansion tank ONLY have the over flow hose from the supply tank into its bottom, with a drain off hose at the cap neck.
I have my "opinions" of how the coolant system "could" work, but I am new to the group so mine should be over ridden by those with more knowledge.
I invisioned the original designed expansion tank as an overflow reserve, thus the cap does not need to have a lower pressure seal and the cap mainly serves as a dust cover and top sealing used.
CORRECTION TO ORIGINAL REPLy
would it hurt if the expansion tank had a lower seal and a pressure setting...maybe. by having a pressure in the expansion tank it would increase the relief of the supply tank.
SORRY for the original erronoius posting
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