Wednesday, May 29, 2013

Front Master Cylinder

This month has been so weak on posts I figure I'd better put up a good tech article and try to redeem myself. I've been slowly combing over Priscilla fixing little bits in an effort to prepare her for new ownership. She'd been sitting for quite a few years and this is not good for motorbikes, thus she needed some love! I've already gone through the carburetor and did a two fluid change with filter as well as fixing a few issues inside the primary. Next on the list is brakes, my least favorite thing of all! Rebuilding a master cylinder is a pretty straight forward operation  and I know most of you don't need me telling you how to do it. Thus is not my intention, but rather to simply highlight a few issues that I've noticed in recent years. Issues like the rebuild kits not containing all the parts needed for a full rebuild.
Included in this picture are all the parts you will need to spruce up your master cylinder. Unfortunately, and unlike in years past, not all of these parts are included in the kit! On the right hand side of the pic is all that comes in a rebuild kit. On the left are all the little parts you'll have to salvage from the old MC. These parts are often quite crusty and may require some effort to clean up. I've found that a bench grinder with a brass bristled wire brush works very well. These parts, especially the little cymbal looking thingy(industry term), are critical to the function and long life of the master cylinder.

This is where the little cymbal looking piece goes in relation to the fluid seal inside the MC. This little seemingly insignificant part that is not included in the rebuild kit serves two very important functions. First, it helps the seal maintain it's shape over time and prevents the spring from cutting through the center of the seal thus deforming the seal and causing fluid bypass. Second, the little protrusion in the center fits into the center of the small end of the return spring keeping the spring centered in the master cylinder bore. This prevents the spring from pushing off to one side and scoring the bore there by destroying the master cylinder. Upon disassembly, I found that this part had not been properly installed, but had fallen out of the seal completely and was trapped between the spring and the bore. Fortunately it hadn't seen enough service there to do any damage. Be sure this item is properly installed and remains properly installed when assembling the MC. Over the years I have rebuilt dozens of master cylinders whose failures were due to misuse or omission of this part alone. I've found that a small dab of approved grease or a drop of brake fluid in the recess of the seal will create enough surface tension to retain this part during assembly.

Another important part of the rebuild is to clean the bore of the MC. This should be done with great care as any large scratches in the bore will allow fluid to bypass the seal under load. I usually use a small brass split rod in my 90* angle grinder and a well worn patch of scotch bright to polish up the bore. I realize a lot of peeps don't own those things so here I am demonstrating an old tried and true hand method. Simply take a small patch of scotch bright and wrap it tightly around the tip of a flat blade screw driver. Rotate the screw driver as you feed it slowly into the bore. If you've wrapped the scotch bright tightly enough around the tip of the screw driver, it will stay there as you rotate the screw driver back out. Think of this as a honing process similar to what would be done to break the glaze in and engine cylinder before installing new rings, only way slower. All you are trying to accomplish  here is to clean any light crust or build up from the bore. If the bore is clean, skip this process. If the bore is severely corroded, this process probably won't save your MC. I use a right angle pick with a good tip to gently remove any crust or corrosion form the snap ring groove. Once you've sufficiently cleaned the bore, coat it with fresh DOT 5 (only if you're doing a complete system flush) and reassemble the MC. Never, never, never mix two different grades of brake fluid!!
The thick felt washer is a moisture barrier. One of the flat washers goes in on either side of the felt and the spring on the piston actuator rod applies pressure that expands the felt creating light friction on the bore and around the shank of the rod. This will prevent moisture, all be it not well,  from invading the bore of the master cylinder. A dab of grease inside and out on the felt, and proper installation will prolong the life of the master cylinder, and when dealing with this old antiquated technology, every little bit helps!
I would like to tell you that I've discovered a great way of extracting and installing the internal snap ring, but these are of the devil and will always be a pain on the butt! Be sure not to discard the old snap ring until the new one is installed, you probably already know why.
I hope this rambling helps you guys out.
Ride em, don't hide em!!


  1. Very nicely written and informational tech article Wes. Don't know how I've missed your blog, now that I've discovered it...I'll be back.

  2. HL, thanks for the kind words. It's good to have you following along.