Not too long ago I looked into the breather tower valve question on my air
cooled Vanagon and as I understand it you should be able to test the valve
in two ways.

First, and perhaps most importantly the diaphragm should be airtight.  Test
this by blocking one end of the valve and gently blowing and or sucking on
the other end (I just cleaned mine up and used my mouth).  If the diaphragm
has started leaking, split or whatever then some air should leak out of the
pinhole in the top.  If it leaks through the pinhole then you need a new
unit.

Second, the valve is designed to close when there is a dramatic pressure
variation.  If I remember right quickly blowing or sucking on either outlet
causes the diaphragm to be pulled down against the spring and thus close the
passage off.  When this happens no air will pass through the tower/elbow
until you release the air pressure.  Play around with it and if your
diaphragm isn't leaky you should be able to get it to block the flow.

Also, if you are careful you can pry the top off of the unit and inspect the
diaphragm and spring inside and then re-glue the top back on without ruining
the tower.

Hope this non-precise description helps someone,

Jonathan Poole
'83 ac westy