You don't say what model 480 you have, but I think they are all pretty similar. On my 480C hoe, the pedal would not depress any, at all. I checked underneath, and where the actuating rod goes through the cover housing, was rusted/corroded, bad enough it wouldn't rotate to activate. I removed it from the tractor, and soaked with penetrate (50/50 mix of Acetone & ATF but use what you have) and got it freed up. I coated where it goes through the housing with never-seize,and it has been working great ever since.
There are a couple springs in there that release it when you tap a brake, or change directions that need to be checked, to see if they are broken, but sound more like mine, and just needs cleaned & lubricated.
Here is a link to Case Construction with an exploded view of the diff. lock a 480.
Official Case Construction Equipment Online Parts Store and Parts Look Up. Parts for Case Equipment and Construction Equipment. Parts Store for Case Wheel Loaders, Case Excavators, Case Skid Steers, Case Original Parts. Look at the bottom of the picture for the #8 actuating rod, and where it goes through cover housing #12. That is where it binds up.
If you remove the little round pedal, then like 6 bolts to remove the cover housing, you can do the clean up at your work bench. There is a jam nut under the pedal, I think it may take a 1/2", or 9/16" wrench you'll need to loosen to remove the pedal. I'm thinking I put a pipe wrench on the pedal to get it loose. You'll probably want another pipe wrench on the rod it's attached to, to keep it from turning, and possibly torquing the yoke on the other end.
