How much pressure should a heater core hold?

Why would you only test that part at cap pressure?

You need to know it will hold MUCH MORE than cap pressure

If you only test it to cap pressure you run the risk of it blowing first.

Also, I have seen MANY MANY caps fail.

They DIDN"T pop open when they should have.

Causing a radiator, a hose or yes even a heater core to rupture

Don't forget to check under water for bubbles.

30+ years in Automotive field

Operating pressure in the heater core will be the same as the pressure in the rest of the car's cooling system, which is determined by the pressure capacity of the radiator cap. If you are testing for leaks, I'd go to twice the normal operating pressure. for example... if your radiator cap is 15 lbs., then test the heater core at 30 lbs.

Hi as CB said it that is the usual pressure as the radiator cap is set at 15 pounds per square inch so the temperature increases above 100 degrees C to 120 degrees C so the car runs more effectively and saves fuel.

15 - 20 psi filled with water.

http://www.classiczcars.com/forums/help-me/48586-h...

Buy a new one. They are not that expensive, it's the labor you don't want to do again!