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.
Anyway so what to do about that piston? You need to restore it to its original size again. Here’s a fancy way to go about that using a lathe.
I’ve also done this in the past by hand on a bench grinder but you need to be very careful if you’re going to use that method. This particular piston comes from an Avid Juicy Ultimate brake. Avid pistons are easy enough to buy from most vendors but Shimano doesn’t have any small parts available anywhere. I’ve seen this same swelling happen to some very expensive Shimano brakes so this fix is particularly useful in those.