Those are simple solenoids that open and close to allow air to flow from the compressor to the locker. They don't have a check valve that I'm aware of.
One of the reasons that it can back up is if the installer didn't grind a return drainage slot at the bottom of the axle tube.
It is common for gear oil to eventually work it's way up to the solenoid. To clear it out just disconnect the air line from the bulkhead fitting on the axle and run the compressor until it clears.
Some people will go as far as to add an oil separator between the solenoid and the air line.
Cut a slot using a dremel about 3/8" wide right there. It allows oil to drain back into differential and get back in the cycle. Only need it on the rear axle as the front axle seals are inboard.
Cut a slot using a dremel about 3/8" wide right there. It allows oil to drain back into differential and get back in the cycle. Only need it on the rear axle add the front seals are inboard.