The picture at the top of this page shows what happens when you leave a master cylinder rotting in the British climate for 10 years or so. The hydraulic fluid turns into a strange jelly-like substance and it's fair to say this is likely to affect the braking performance. I can't verify that because my calipers were hanging from the front of the bike by 2 lengths of string, but anyway let's assume we want to rebuild the whole thing.

I stripped everything down to individual parts, cleaned and scrubbed it for hours and applied a few coats of Simoniz Tough Black Satin paint to the caliper body.


Before refitting the plastic fluid reservoir to the cylinder body, it's a good idea to replace the o-ring. The original will be very old by now and it's likely to leak fluid if you just refit it. Before fitting, I used a small rotary brass wire brush on a Dremel to clean out the groove that the o-ring sits in (always use brass brushes because they don't damage the alloy).

From this point onwards, you have to keep everything surgically clean. This is your front brake and it will save your life, but only if you rebuild it in absolutely spotless conditions.

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: 3mm

Inside diameter: 44mm

Outside diameter: 50mm

Here's the new o-ring fitted to the cylinder body:


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. Use 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.

You will also need a master cylinder rebuild kit, a screwdriver and some snap ring pliers. Here's a picture of my kit of stuff, ready to start work:


Here you can see the rebuild kit and the cylinder laid out on the table, in (almost) the exact order that the parts will be inserted into the master cylinder. I say "almost" because actually the big rubber sleeve goes on last, but you get the idea:


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 next picture shows everything ready to insert into the master cylinder, except that I've shown the rubber boot fitted loosely in place for reference (you don't want to fit that yet). Put the spring in with the wide end first and the narrow end pushing on the rubber seal.


Then insert the spring, plunger and retaining washer:


And then the circlip that holds it all in. This is where the snap ring pliers are needed, because the circlip is deeply recessed and there's no room for conventional circlip pliers. I couldn't take a picture of that because I didn't have 3 hands, but after the circlip is fitted you can slide over the protective rubber boot.


Double check that the seals are all seated well and that the circlip is fully engaged. You should be able to push the plunger in and have it slide smoothly back under spring pressure.

Next you can refit the reservoir. A little dab of thread locking fluid on the screws is a good idea here:


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.

Then you can cap the reservoir with the rubber diaphragm, the retaining ring and the reservoir cap. These are available as a kit of 3 parts from Mikes XS:


Nice, eh?


I used some nice ACF-50 grease to protect the rubber and alloy from corrosion. I also fitted a new lever return spring (you can see the Yamaha part number on the bag) because the original was a twisted mess:


Fit the brake lever with its pivot and adjustment screw, fit the brake light switch and we are good to go:


There's a nice rubber boot which goes over the lever for weather protection, but I haven't got one yet so I can't post a final photo of the finished article. If you've followed these instructions and kept everything properly clean then your brakes should keep you out of the path of idiot car drivers for years to come.