If you've got the bits and pieces lying around to do it, you might connect a pressure gauge to one side of the heater core, and a valve through which you can apply compressed air through the other. If you can do that, put 15PSI to the heater core, then close the valve. The gauge should not drop at all in two minutes. If it holds up at the initial value for two minutes, the heater core is not leaking and you can look elsewhere.

Just a thought.

 

Temporarily bypassing the heater core as mentioned.. How bout Bypassing it for the summer?? any problems with doing this??
I just aint got the money to fix it right now and mine needs a core ,,

 

It won't hurt the truck any. It's not the best idea for an intact heater core because the coolant that remains in the core will sludge up, but if yours is already shot and will never be used again, no worries.

 

Well I cheaped out and bypassed mine today ran across a hose barb cupler that fit right in .2 hose clamps and 5 mins..,. I hated to do it but funds are short..
Wish I could figure out a way to afix that blend door into a single position,,

Hope my defrost will still work..