A while back, I mentioned a leak in the passenger compartment heater of my 
'84 Vanagon (I think I mentioned it).  Anyway, this supplimental/auxiliary 
heat exchanger has a black plastic housing, and includes what appears to be 
a white plastic bleeder valve.  My leak is near this bleeder valve: it 
appears a PO tightened it too much and cracked the plastic housing.  Now if 
there is coolant flow to this heat exchanger, there is coolant leaking from 
it.

My question: Will one of those leak-sealing wonder-additives plug this leak 
and allow me to use this heater?  Or should I just close off the flow of 
coolant to it and provide blankets for my passengers?

Replacements are probably more $ than I intend to invest.  Any other 
suggestions?  Thanks in advance.