Just got done doing that, along with new front upper balljoints.

 

If the cap isn't completely closed it'll make the overflow spew everywhere. Learned that the hard way. Thought it was completely shut but I had a quarter of a turn left.

 

Was not spewing everywhere, just overflowing the expansion tank.

 

If the cap will not hold pressue the coolant will boil inside the system. 50/50 mixture of coolant boils around 230 degs. Your whole system stays under 230 deg but areas around the heads get MUCH hotter than 230 once the car is all warmed up. Class was a looong time ago but I think you raise the boiling point of the coolant 2-3 degrees for every one pound a pressure. Boiling coolant can trap gasses around the head and cause head gasket failure. In the 80's defective caps where so common we tested all new caps before they where install.

Now when I say common I am talking one in a 1000 bad caps, but when you add in the fact the bad cap can cost you a motor and the test takes about a minute if you already have the tester out. Also every car that had a coolant system issue got the Cap tested. It is part of the coolant system check up.

The older caps where 16 PSI, I can not imagine 2 PSI more would hurt anything. Infact from the testing I had done many of the 16 PSI caps did not release until 17-18 PSI. Any cap that did not make it to 15 PSI was discarded.

 

I've bled her till she can't bleed anymore.

 

Follow up, since I replaced the lever style cap with a regular cap I have not had any more issues. So either I got a bad cap or those lever style caps suk...lol