Proportioning Valve Rebuild?

[cLaWz]

Is it possible to disassemble and rebuild one of these are are they pretty much fool proof from the get go?

Was doing a final check before going to get my truck inspected today and found my parking brake wasn't working correctly. Never used it since I picked it up so I never noticed it. Tried bleeding the brakes but got no fluid out of the rears... hit the proportioning valve with a test light and it lit up so the valve is "popped". Cracked open the fronts, and closed the rears and pumped the brakes but I cannot get the valve to reset.

Do they generally go bad over time or should I begin digging into the rear brakes more to see if there is some other problem? Not seeing any fluid leaks anywhere at the rear wheels.

Figured I would ask before tearing stuff down even more.

Thanks