Yep, broken 'O' ring

Here's the little perisher. I know a weak point when I see it so I purchased 3 spares. Now 2 spares.

Fitting the new one requires a 3D printed tool and a lot of brute force. They get hung up inside 2 out of 3 times, requiring fishing around with tweezers before trying again. Still, better case than fixing the Y limit switch.