My AC guy was out today to do the one year warranty inspection for my
house AC system. He's a real chatty guy and loves to talk AC repairs.
I told him the "Stormin Norman" method of determining if slow leaks
were in the low side versus the high side. He said the SN theory made
no sense to him.

He went on to say that in his experience the vast majority of slow
leaks are on the low side so perhaps the reason SN thinks his method
works is because he almost always gets a zero reading (cause the freon
is all gone) and it's almost always a low side leak. So it seems like
SN theory works but its really coincidence that makes it seem to work.

He said the reason for most leaks being on the low side is because
it's usually in the inside coil (located on the low side of the
system) because that's the coil that gets constantly heat stressed
plus it's the coil where the frame rusts away which lets it expand and
contract more as it gets old and the frame gets rusty. He said he
sees it especially in heat pumps that switch from running the inside
coil at 140 degrees while heating and when they go into defrost mode
they immediately get flooded with cold freon and go to 24 degrees. So
they get a lot of expansion and contracting from large swings in
temperature. The outside coil doesn't see nearly as big of
temperature swings. He also said he thought that since they are
usually wet from condensate and often made of dissimilar metals there
may also be some electrolysis going on that generates thin spots and
eventual leaks.