I use a shurflo 4008, all 5/16" tubing, 2 main runs totaling about 90 taps.

when looking for squirrel chews, I run my hand along the tubing and inspect the slightest abnormality that I may feel. I find it much easier to feel them than to see them. (I'm not sure if they hiss.....I wear hearing aids due to high frequency hearing loss but they are programmed to amplify speech patterns at those frequencies and not other sounds.....so I'm not sure I'd hear hissing if it occurred.)

Anyway, I have always found the squirrel chews right at trees, so that is where I go first. They can be the smallest nick....1/8" long x 1/16" wide, and cause problems. I wrap electrical tape for a temporary solution unless the hole is large. Usually if there is one nick at a tree, there are others.

As CampHamp said, if you find a column of sap in the tubing, look on the pump side of that point....a fitting, a drop.