Remove the screws connecting the plunger tube end cap to the plunger tube using a Phillips #00 screwdriver.

Pull out the plunger to reveal the worn-out O-ring on the plunger head.

When reassembling, a strong spring will increase the risk of stripping the screws and difficulty in reassembly. Use a screwdriver slightly bigger than the slot and push hard into the end cap for the best results.

You can tell if your O-ring is worn-out when it swells past its advertised dimensions, allowing it to sag or slip.