I have a rear underseat heater  on an 85 Westfalia that has made a complete
fool of me.  I have a small leak that arises somewhere in the core or at
the valve and each time I think I have it fixed it comes back to haunt
me.  Finally took it to a radiator shop and they recounted their own
problems with them.  It seems that each piece of tubing is embedded
discontinuously so that leaks can arise at either lower or higher
pressures  where the pipes enter the end manifold and there is nothing that
can be done about it.  I have had that heater in and out 6 times.  This guy
said that he has watched cores that are perfectly sealed at 30 PSI and leak
like sieves at 15 PSI.    It will never do that to me again----I beat it to
death with a sledge hammer.   Please don't let it make the same fool of
you.   It used to be that as a young man I had endless patience for such
nonsense, but no more.  I find that the sledge comes out more and more
frequently now, may eventually take it to the whole vehicle.  I am not
proud of that kind of behavior, but not too ashamed of it either.