I had the same problem with my spare under my 2008 Chevrolet Silverado recently. We were getting ready to move from Michigan to Oregon and I was planning to pull a UHAUL trailer. So I checked the pressure in the spare and it was low. When I added air I could hear it leaking out. So I tried to lower it and couldn't. I followed the instructions in the manual to deal with the secondary latch to no avail. The garage that was going to rent me the trailer said it is a common problem and that the mechanism would need to be cut off and replaced with a better design. So that is what I had them do. Cost about $300. The rim was rusty so the guy at the shop did some grinding and then reinstalled the tire and pumped it up. Seemed to hold air. Fortunately we didn't need it on our trip west.
