[Antennaware] Testing coax w/MFJ 259

[Arthur Trampler]

Hey Gang,
 
Some of you have followed my tale regarding the Hy-Gain AV640.  Hy-Gain suggested that rather than saturation, it would be moisture in the coax or a bad solder joint on the 40 meter coil.  I was not able to force an SWR climb on 20 meters after 10 minutes of nearly constant "CQ" at 1.5KW out (dead band).  Again, this is fed with about 125' of Davis RF LMR400, so at 14mhz I still have a good deal of RF at the antenna.
 
I lowered it, resoldered those connections but rushed it...thunderstorms coming in...and will re-do it.  However, Steve Davis was good enough to respond to a question I sent him and agrees that moisture seems unlikely if it only occurs on 40 meters.  He suggested testing the coax with something like an MFJ259 or similar.
 
My question: what would I be looking for to indicate a problem with the coax?  Would it be a change in the velocity factor?
 
I'm sure I can borrow one locally, but unsure what tests I ought to run to check the coax and if any will reveal the likelihood of moisture in it.
 
My last step would be to see if I can borrow 100' of RG8, 213, 9913/whatever to run from my surge arrester/ground to the antenna in place of what I have.  I guess I am trying to give Hy-Gain the benefit of the doubt that the problem is not with its matching unit.
 
So the question: What tests are recommended to test coax integrity and specifically the likelihood of moisture in it?
 
Thanks,
Art, KØRO