On the plastic compressor hoses, if the spring isn't in place, sometimes the hose will crimp near the nipple, and a) not deliver enough pressure and/or 2) develop a leak at the site of the crimp. Here's what I did to fix that, and prevent it from happening again.

Step 1: Remove Nipple, Repair Hose

I cut the hose above the crimp, and removed the remaining piece from the nipple.
You may have to use an x-acto knife or a cutting tool to cut through the hose piece; it's fairly tough.

Step 2: Reinsert the Nipple Into the Repaired Hose

You may have to use a couple of pairs of pliers to force these pieces fully into place.

Step 3: Affixing Spring Into Place

Keeping the spring in place is what keeps this hose from getting crimped.
One way to make it stay is use a dab of Shoe-Goo. Thanks to my wife Carolyn for this tip!