My 4905 has a large check valve right at the main air tank in the line above the batteries. About 1" dia. They list a repair kit, probably no longer avail. I replaced mine when I went over the complete air system last year. Everything is rebuit or replaced. The tank btw is split with a check valve between them. You remove a cap on side of tank to service it.
Here are pictures of mine...The big line lower left next to brass fitting is main checkvalve (silver grey hex). That line goes to back of coach to air compressor. ...