From MrSeas:

Replacing O-Rings in push-in air connectors can be a much cost fix than replacing the fitting.

Go to the dealer and get the replacement parts for the push in connector. There is an "0-ring" and a brass collet that are replaceable. The O-ring is a greenish-blue color.

Here are the Volvo part numbers:
1/4 O-ring 85108484 price $0.48 (each) come in a bag of 10
1/4 brass collet 85108476 price $ 2.86 (each)
3/8 O-ring 85108485 price & 0.56 (each) come in a bag of 10
3/8 brass collet 85108477 price $ 3.10 ( each)
1/2 O ring 85108486
1/2 brass collet 85108475

To replace the parts is pretty straight forward.
Please release all the air pressure in your system before attempting this repair!

1. Depress collar on fitting and remove line.
2. Remove brass collet. I use a pair of needle nose pliers.
3. Remove O-ring. Sometimes a bit of a challenge. I use a set of " dental picks" one straight and one with a 90" bend at the end. Use the picks to remove the O-ring. Most times it will be dry rotted and come about in prices. Make sure to remove all of the pieces. Insert new O-ring and be careful not to damage it.
4. Insert collet. Most times I will use the old one if it is still in usable shape. Most times the o-ring is the cause of the leak.
5. Trim off the end of the line before inserting into the "rebuilt" fitting. I like to do this when ever I have enough slack in the line. Sometimes it's not possible. When inserting the line into the fitting press the line in until it "bottoms" out in the fitting before you pull back on it to lock it in.
6. Pressure up your system and check for leaks. Sometimes a little wiggle on the line after it is pressurized will help it seat.
After this less costly repair attempt you may still have to repair/replace your pass-thru.
Good luck!