Few things are as annoying — not to mention messy — as a leaking heater core. Getting coolant and vapor all over the new interior trim materials is a huge nuisance, not to mention having to take things apart to repair or replace the leaking core.
Whether you are using an old core or a new replacement, it pays to test it for leaks. Filling it with water won't do the job, however, since most heater cores only leak under pressure.
There's an easy way to test your core, and one that requires no equipment. All you need to do is press your thumb over one hose outlet and suck on the other one (you might want to clean it off first!) Suck on the pipe firmly to create a substantial vacuum, then hold for a few seconds to see if the vacuum goes away. You'll know if it does, and that means the core will leak.
How effective is this test, you might ask? Well, at sea level you will be able to create a vacuum of nearly one atmosphere, which is equal to around 14 psi. That just happens to be very close to the operating pressure in your cooling system. If the core can take a vacuum, it can take the cooling system's pressure.