R12 Schrader Valves

Has anyone restored the Schrader valves on their AC system? I unscrewed it and collar came up off the threads but the pin in the center did not allow it to come off (it was mushroomed). Tried to tap the whole assembly from the back side where the sealing plunger is and it was not moving... How do these Schrader valves come apart? Are there o-rings you can replace?

Jim