Rebuild the Master Cylinder on a Triumph Twin


| 4/14/2014 12:35:00 PM


Tags: may/june 2014, classic triumph motorcycles,
New for old: New stainless steel master cylinder barrel and piston above old and tired original.
New for old: New stainless steel master cylinder barrel and piston above old and tired original.

The disc brake fitted to Triumph twins starting in 1973 was generally regarded as a great improvement over the previous drum brake system. Powerful and reliable, it was good enough that Triumph used the same setup right up to the end of production in 1983. But that means even the newest disc brake-equipped Bonneville is more than 30 years old, and if you're of a certain age you'll remember the old call to action, "Never trust anyone over 30." That "wisdom" probably applies more immediately to brake hydraulics than people, and if the front disc brake on your Triumph twin has been less than useful lately, it may be time to have a look at the condition of the master cylinder.

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A major issue in brake master cylinder failure is the fact that brake fluid is hygroscopic, which means it draws moisture out of the atmosphere. Over time that moisture makes the fluid gummy, and the moisture corrodes the master cylinder walls, reducing the effectiveness of the braking. Further, a great deal of heat is generated every time the brake pads grab the brake rotor, and some of that heat is transferred to the brake fluid. Brake fluid is specifically formulated to have a high boiling point because if it turns to vapor it becomes highly compressible. Moisture in the system significantly reduces brake fluid's boiling point, introducing the potential for a dramatic if not complete loss in performance when your brake system gets hot.

Fortunately, rebuilding a Triumph master cylinder is actually easier than it used to be, as most parts suppliers now provide a complete assembly that eliminates the tedious task of replacing individual parts. Being less than completely fluent in all things Triumph, we didn't appreciate this before we began our rebuild project, resulting in our swapping out our master cylinder the old fashioned way, piece by piece. Certainly, there's value in knowing how to work your way through the process step-by-step, but you can save yourself some trouble by buying the complete new master cylinder barrel assembly.

Here’s our Triumph twin master cylinder before rebuilding. It wasn’t leaking fluid, but the handle was soft and pulled farther than it should.
Here's our Triumph twin master cylinder before rebuilding. It wasn't leaking fluid, but the handle was soft and pulled farther than it should.

ron
1/23/2016 5:20:50 PM

This would have been an excellent step by step "how to", but you failed to demonstrate/display the most important steps - how the internal parts go together - Even after waiting for parts to arrive and time to get back to the task any dumb bunny can install and bleed the system... - but to remember the exact order of installing the internal parts was completely left out of this "step by step" article - how unfortunate...


davidp
9/14/2014 12:59:18 AM

Please use only Girling-approved,DOT 4 brake fluid (Castrol LMA, for example.) Otherwise you'll be repeating this procedure often, when the cheap DOT 3 fluid eats your seals. While you're at it, trash that cheap brake switch and install a real hydraulic brake switch. Not original equipment, but they actually work.





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