I replaced all the vacuum elements. I did not replace the changeover valve nor the heater core. Looking back, I should have replaced the heater core since it has to come out anyways although mine is still good (knock on wood) after all these years...
If you do it yourself, change your instrument and a/c vents light bulbs and specially the evaporator drain hoses on each side of the center console. They will get destroyed when you take them apart. You can get a better hose at home depot...You will know what I mean when you get there...

Good luck!

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J.H.
'86 300E