I replaced the heater core in my 2000 Civic HX and got a brown puddle for my trouble. My old one didn't do that, so the new one is actually worse!
Testing seems straightforward enough, two rubber corks, a Schrader valve, and a bike pump. Drill a hole for the Schrader valve in a cork, plug both ends, and pump to the pressure rating on the radiator cap.
Had I done this in the first place I could have sent it back for a warranty replacement before I installed it!

 

Hey guys, I thought that I figured out how to properly test a heater core before installing it. I trimmed down a Schrader valve to fit in one hole, inserted a rubber plug into another, and used these two mending braces, bolts, washers, and nuts to keep the Schrader valve in the hole. The cooling system is rated for 16 PSI, so I took the bike pump to 16 PSI, set a timer for an hour to ensure that the core holds pressure, and just a few minutes later the pressure was gone.
Is the problem with my core or my rig?
Oh! How do I remove a rubber stopper from a hole?! My rig pushed it almost all of the way in!

 

your likely going to find all kinds of leaks if your testing with air only. when they pressure test a cooling system they add the pressure ontop of the antifreeze. also why does your coolant look like my oil during an oil change. if you have a cracked head and your oil and antifreeze are mixing thats not good. could also be over pressurizing your coolant system. as for your rubber stopper problem dunno they are pressure tested before being sold so assuming you didn't buy one from a junkyard id assume something was loose and it leaked.

 

The stainless steel bypass valve that I installed rusted out. I flushed out the engine block.
Heater cores are pressure-tested at the factory, but a friend that works at a parts store says they have a high rate of warranty return.