Heater Core/Valve Leak

[Ceeker]

visual test for leak:

I have encountered this problem a good number of times. In my history of these cars rarely do the heater cores give problems unless the model questioned did NOT get the SIB from bmw when they had exploding cores. Once the SIB took effect core issues were rare.

look for sweat around the heater tap - most likely the issue with your leak. if you end up changing that, change the 3 o-rings as well.

[Pennyw1se]

The previous owner bypassed the tubes at the firewall and I am assuming this was done because of a valve or core failure. This hasn't caused any overheating issues so I have left it alone.
What is the best/easiest way to test for a leak without reconnecting hoses? Could I run water through both pipes at the firewall with the ignition off and look for a leak since the valve is open at this point, or does there need to be pressure in order for the leak to appear? Is the heater core essentially a small radiator that can be removed then checked for leaks by filling with water and checking for leaks somewhere?
I have a spare parts car that I may be able to swap parts with but I'd like to bench test before I start swapping so I don't waste my time on useless parts.

[Nürburgring]

I believe you could do a heater core flush and you'd find out if you have a leak or not...

Buy a kit for $4 from autozone and watch the ChrisFix youtube video on how to do the flush...the only difference will be that the water you run during the flush will be cold garden hose water instead of near boiling coolant fluid. Under extra pressure, you may have a leak whereas with cold water and low pressure you may not show it.

http://www.bing.com/videos/search?q=chris+fix+heater+flush&view=detail&mid=E2A575FF3BFACF223B7FE2A575FF3BFACF223B7F&FORM=VIRE1