Ever had a hydraulic brake lever that just won’t pop back? Mountain bike brake companies did something smart with their master cylinder pistons to avoid scoring the aluminum master cylinder bore. They made their pistons out of plastic. The one downside of the master cylinder piston being made from plastic is that plastic can absorb fluid and swell. Despite polymer suppliers claiming that their material is ‘dimensionally stable’, the real truth is that over time, it can absorb a little bit of fluid and grow slightly. So what does that cause? It means that one day there may come a point where you pull your brake lever in and it doesn’t come back out on it’s own because the now swollen master cylinder piston is stuck inside the master cylinder bore.