After awhile the O-rings wear out. To repair them is just a simple remove and replace with new O-rings. Coat the new O-rings with air tool oil before reinstalling them.
For a “quick” fix. After starting the engine, depress the clutch and move the transmission into first or reverse. There should be a sweet spot where it will stop leaking. Hold the shift lever in the sweet spot till the air builds up fully. Once the air pressure has built up, move the transmission into neutral. Often, once the air pressure has built up and the truck has warmed up the O-ring will stop leaking. The leaking O-ring will still have to be replaced, they don’t get any better.