The Master Cylinder

This is the undertank master cylinder off from a 1976 R75/6. It was leaking out of the piston in the rear from bleeding the brakes without blocking the handlebar and so it overextended in the bore.

Remove the reservoir

First step is to remove the reservoir. There's an allen head bolt that holds it on inside the bowl.

Bowl removed

This is what it looks like with the bowl removed.

This o-ring is the cause of quite a few leaks

This o-ring is the unfortunate cause of most leaks from the master cylinder. It seals the reservoir from the rest of the cylinder. Unfortunately you can only get a new one with a rebuild kit, so do the whole thing while you're at it.

Testing the brake switch

Easy way to test the brake switch. These rarely fail unless they're clogged. First hook up your ohmmeter and make sure that there is infinite resistance.

This switch tests out fine.

Then carefully take a finish nail, and put it head first in the hole in the brake switch. Push down hard - I use a screwdriver handle to help me so I don't hurt my hand. Be careful you don't ruin the switch, but you should see the circuit open up.

Next remove the circlip holding the piston in the bore.

This picture shows the circlip (or "Jesus clip" as my father calls them) holding the piston in the cylinder. You need to carefully remove it.

"Custom" circlip pliers

I made a set of "custom" circlip pliers by grinding the tips down so that they were smaller for working with these cylinders.

Holding back the piston in a vice

You can carefully hold the piston back in the vice so you can remove the circlip, but I prefer to stick my thumb over it for disassembly as it will easily push the piston out and make disassembly easier.