I'm curious to know if others have found high failure rates with radiator caps not holding the prescribed pressure, or do most just assume the cap is functioning, and place it into service without pressure testing?

 

I only purchase one brand of radiator cap - made by Stant. They make excellent products. I've never tested any.

 

I've never tested one either. And I've never had one leak coolant, or had an overheating issue, so really no reason to test. FYI my car has an aftermarket overflow reservoir and currently has a 7 psi cap. No issues with excessive coolant in the reservoir that would indicate it's not holding 7 psi.

 

How is your cap failing? Do you have an overflow tank?

 

I've only had issues with the rad caps only pushing into the overflow, not pulling back out. Yes, they are the proper overflow style rad cap.

 

A buddy tests all the caps for his cars, he has a lot of cars in the collection, he said over half of the new caps he tests, don’t hold pressure.

 

^^^^^^This^^^^^^

Exactly what my tests conclude!

To expect a new, out of the wrapper cap to function correctly is folly.

 

So what is the failure? As in what pressure does a 15 psi cap hold?

 

In the ’70 service manual it states the cap should hold at least 14psi, if not replace the cap.

Regarding my initial question, I tested 4 new, out of the box, RC-15 caps, and had 1 rebuilt, RC-15 cap on hand . Only two of the 5 caps passed pressure testing. Apparently, radiator caps like voltage regulators, condensers and other electrical goods should be suspect and tested prior to service nowadays.

 

What pressures did they fail at?

 

All in question failed to hold 14psi; the needle slowly slid south into the 6-10psi range.

 

Did you try rotating your setup 180 degrees to see if they slid north?

 

I tried every thing I knew to do using my tester. Today I gathered up several of the caps and visited my local auto shop, and our old school radiator shop we are lucky enough to have (in business since 1947), both caps failed with results very similar to what my Stant tester gave me.

 

I was subtly pointing out your disparaging reference to The South.

 

Disparaging? You do realize I'm a Southerner?

Deteriorate or decline, as in The stock market is headed south again. This expression is generally thought to allude to compasses and two-dimensional maps where north is up and south is down.