the power of instruction bleeding your brakes

Wed, Sep 10, 2008

I’ve got a task to perform which I’ve never done before. Bleed my mountain bike disc brake. On a trip to the ‘gorms I was cycling out from Red Bothy and the front wheel locked solid. I had to stop every couple of minutes to let the caliper cool down and prise the pads from the rotor. I’ve since found out it’s because I haven’t changed the fluid in ages and you should ideally bleed them every 6 months. The slightest rubbing of the pads on the rotor causes the calipers to heat up, which in turn transfers heat to the brake fluid, which being rather old and hydrophilic by nature, had been contaminated by minute amounts of water. The water then heats up and expands, causing the pressure in the cable to increase, pushing the pistons out and the pads to contact the rotor, which increases the heating effect and creates a vicious cycle, quite literally. The net effect is a wheel that is increasingly difficult to turn and so I had to stop every couple of minutes all the way back to the Linn of Dee. I suspect the crossing of the Geldie Burn on the way to and from Carn an Fhidhleir and An Sgarsoch had made the problem worse as the water was knee deep and the caliper had been submerged. The cycle back to Red Bothy was fine but in the morning, the brakes had become locked up as soon as I left for Linn of Dee.

Anyway, the point of the story is, apart from saying you should properly maintain your disc brakes, that being shown the right way to do it by an expert is worth a thousand books and internet articles. I found this video on the Hope website and it more or less shows everything I was expecting, from reading around the ’net, apart from that one little procedure at the end. Rolling the diaphragm back onto the reservoir. If you just plop it back on you’ll trap air in the system and the brakes won’t work and I’ve would prolly have found that out halfway down a mountain track! Seeing an expert do what you’re trying to do is definitely worth a thousand internets. Here’s the video. Hopefully you’ll find it as useful as I’m going to!

There are more useful videos here.