Propane tanks are fit with a valve controlling the flow of the gas. A hose is attached to the male end of the valve. Inside the coupling mechanism of the hose, the female receiver unit, an O-ring is housed. The receiver fitting of the hose and the male end of the propane tank valve are metal. When the two are coupled without the O-ring, the fit isn't airtight. The purpose of the O-ring is to prevent propane from leaking out of the junction between the male and female fitting. Cracked and brittle O-rings leak.
Wedge the tip of a knife between the O-ring and the inside of the female receiver. To dig the knife under the O-ring, slide the knife over a 1/32-inch lip that holds the ring in place, then slide the tip under the outside of the ring. Damage to the O-ring isn't important because you're replacing it.
Reach into the female receiver with the needle nose pliers once you have the knife wedged under the ring. Grab the ring and pull it out of the coupling. The O-ring may pop out of the receiver when the knife is wedged under it or it may come out enough so that you can grab it with your fingers. If not, use pliers.
Clean the inside of the coupling with rubbing alcohol and a cotton swab to remove any residue from the fouled O-ring. Rub the replacement O-ring with gasket sealant. Push the O-ring into the coupling. It's critical to-maneuver the O-ring to the back side of the 1/32-inch lip. To do so, you may need to use needle nose pliers. Be careful not to damage the O-ring. Visually check the O-ring to make sure it's not twisted or bunched. Check with your finger, also. You should feel a perfect circle inside the coupling.