"Is the over flow res cap vital to the system?" Need a tank before doing any other work. But tank will work w/o the cap.
You don't need all of air out but only to make sure lower Tstant is covered with coolant so the Tstant and sensors/senders read coolant temp.
Bad cap or bad areas on the housing can cause even new caps to fail to seal But Good parts can fake problems because can take 1 to several heat cycles to purge air out of system.
See my Cave, Coolant Fill
and rest of cooling section.
Then Read again...
Weak/bad or no coolant can wreck the system from inside and still look ok otherwise.
Example, If rest of system is good can still have plugged tubes in rad and/or heater core that kills coolant flow.
------------------Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)The Ogre's Fiero Cave
(It's also at the top and bottom of every forum page...)
[This message has been edited by theogre (edited 03-01-2015).]