sounds like you really have nothing to worry about. if the cap won't vent, pressure will go too high, but i think your new hose is just a bit different from the old hose, and your new t-stat just opens later. relax, you did a good job even w/out the flush. if the old coolant wasn't nasty dirty and acidic, worry about it next year.

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1984 300TD "MAX" 303K+ still going...
fast '70 Chevelle 200k+
home built Shovelhead chopper