How to Rebuild a BMW Front Brake Master Cylinder

Follow along as Keith Fellenstein repairs a brake master cylinder.

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by Keith Fellenstein

Our project for this issue planned itself. Editor Hall picked up this 1988 BMW R100RS late last summer. At some point over the winter, he noticed a spot of fluid on the floor and quickly figured out it wasn’t engine oil, but brake fluid. The front brake master cylinder was weeping fluid and it was dripping into the inside of the fairing, and eventually to the shop floor. Not good. And since seals don’t heal themselves, for this How-To we will be rebuilding the master cylinder on this R100RS, and we thought you, dear reader, would like to see what goes into the fixing, as there are a lot of these Eighties Beemers still on the road, and frankly the process is very similar for most master cylinders.

Once we removed and disassembled the cylinder, we found that it had developed a decent leak at the plunger. Any kind of leak in the brake system is concerning, so we’ll get this sorted before it becomes a safety issue. We’ll be using a parts kit from Euro MotoElectrics and brake fluid by Spectro. As always, it’s good to have a factory shop manual on hand just in case. Let’s jump in and get started.

An upclose photo og short bars on a motorcycle

1. I’m sure there are fans of these short bars, but I’m not among them. They make it a real challenge to get the bike up on the lift. I think these might even be shorter than the stock bars for this model.

A black master cylinder reservoir cap

  • Updated on Aug 17, 2021
  • Originally Published on Jul 28, 2021
Tagged with: brake master cylinder, how-to, motorcycle repair
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