Words and pictures: Alan Dowds
WE did an overhaul of the seals in a brake master cylinder the other week – so here's how to do the same job down the other end. Caliper seals get a much harder time than the master cylinder though, and need changing much more regularly. Grit, dirt, corrosion and water are all constantly attacking the small rubber rings which keep the brake fluid inside the caliper. As the pistons move in and out, they wear the special sealing rubber, and any corrosion on the surface of the piston can tear the seals.
Here's how to change them.
We're working on a sliding, two-piston caliper here, but the principles are similar for most calipers. The first tricky part is to get the pistons out. Remove the caliper from the bike, but leave it connected to the hose. Take out the pads and any sliding mounts, then carefully pump the brake lever to push the pistons out. If one comes out more, then use a piece of wood (or the pads) to hold it in place, and pump the others out evenly. Take care to get them all to the same point – once they are nearly out, you can manoeuvre them out the rest of the way. Once one comes out, you obviously lose pressure, and can't push the other(s) out any further.
Four- and six-piston calipers are trickier of course. Go steady and you'll get there. You can buy brake piston pliers which lock inside the piston and help you pull it out, if you get really stuck. With some two-part calipers, you sometimes need to unbolt the halves – make sure you have new seals for the passages between the two parts for the rebuild.