If your rear master cylinder looks like the one at the top of this page (mine), then you will probably want to renovate it. This particular one had been standing outside, unused, for about 10 years - and needless to say it was a mess.

So the first thing to do was strip it down to its component parts and clean everything thoroughly. What's left is the master cylinder body, which I have cleaned, scrubbed and ultrasonically agitated so that it's spotless. I also applied a coat of polyurethane lacquer to it, but you don't really have to.

Next it's good to replace the big o-ring that seals the plastic reservoir to the cylinder body. It doesn't show up well in the picture, but you can see the old one and the new one (in the packet) next to each other.

This o-ring isn't listed as a separate part by Yamaha - they only sold entire master cylinders - but you can buy one easily enough. I got mine from Simply Bearings online and the important dimensions are:

Cross-section: 2.62mm

Inside diameter: 44.12mm

Outside diameter: 49.36mm



When putting these things back together, it's really important to use a lubricant, otherwise you run the risk of damaging delicate rubber parts. Traditionally, brake cylinder components were assembled with a special red grease that is formulated not to attack rubber or spoil brake fluid - but this is not so easy to get nowadays as most people use silicone spray:


You spray a little bit on the rubber seals and they become amazingly slippery. It's easy to wipe off any excess and it won't contaminate brake fluid at all.

Once you've inserted the new o-ring into your surgically clean master cylinder, you can put the reservoir back in:


It's a tight fit, but with the silicone lube it should pop right in.


I haven't found a way of making the reservoir look nice and white again. They all seem to go yellow with age, so if you have any suggestions for fixing this then let us know.

After re-inserting the reservoir you have to screw it back in, using the little plate and 2 flanged screws. Mine have been stripped and zinc plated because the originals were rusty. I also used a little dab of thread lock fluid just to make sure:



On this particular master cylinder there is a threaded plug and washer at one end. Later types don't seem to have that plug. Again, I zinc plated it because it was rusty and I used a little thread locking fluid just to avoid having my brake fluid fall out and go over the tyre.


Next you'll need a master cylinder rebuild kit. Here's the one I used:


Here you can see it laid out on the table, in the exact order that the parts will be inserted into the master cylinder:


Before you insert the parts, there is a little assembly job to do. The rubber seal on the left has to go over the plunger shaft, so that it is seated around the locating ridge. At first it seems as if it won't fit, because the hole in the seal appears too small to go over the flange on the plunger, but with a little of the silicone spray you can ease the seal over the end. Push it on sideways and turn it through 90 degrees, then it goes on surprisingly easily.


The picture above shows everything ready to insert into the master cylinder. Put the spring in with the wide end first and the narrow end pushing on the rubber seal. Then insert the plunger, the retaining washer (C - shaped) and finally the circlip. Double check that the seals are all seated well and that the circlip is fully engaged.


Finally fit the rubber boot over the master cylinder and locate it in the groove on the plunger:


Finally, just insert the rubber diaphragm, the retaining ring and the reservoir cap. In this picture my retaining ring looks a bit like the surface of the moon, because it was heavily rusted. But with a bit of zinc plating it's probably OK now. I hope.


I couldn't find any of the correct fixing screws, so I just used some small stainless set screws and washers. Looks OK to me.


If you've followed these instructions and kept everything clinically clean, you should be good for many more years of happy braking.