I made this at home after seeing that they are 15-20 dollars online and even more expensive at stores. I made it for around 6$ and had some of the stuff around the house. It worked perfectly for my compressor.
Step 1: Parts Needed
2- brass hose barbs (whatever size you need for air hose you're using)
1- brass coupler (same thread size as hose barbs)
1- pencil eraser
1- rounded thumb tack
Step 2: Determine Air Direction
Thread one hose barb into coupler. I made an arrow on mine to show air flow.
Step 3: Make Air Restrictor
Put pencil eraser into the spring and insert rounded thumb tack.
Step 4: Glue O-ring
Super glue o-ring onto other hose barb. Make sure it is slightly smaller than the rim of the barb.
Step 5: Assemble Parts
Put spring into coupler and screw in other hose barb. Make sure the thumb tack's rounded edge is facing the o-ring. This will create a seal when air tries to return to the source. You will have to experiment with the spring and torque of the hose barbs depending on the amount of air you are using. Stretch or compress the spring if needed. I tested mine up to 150 psi with no backflow. This is for inline application, but replace one barb with a nipple if you want to direct-thread it to a tank or other application.