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.
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| 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. |
This is not a particularly difficult project: If you can bleed your brakes, you can rebuild or replace the master cylinder. There is only one tricky adjustment to make, and we’ll show you how to do it. This project should take only a couple of hours, but as always, budget plenty of time and, more importantly, take your time. You don’t want to hurry a job as important as your brakes.
Just about any of the British bike parts houses will have what you need. British Cycle sells the seal kit for $22.90 and a stainless steel replacement barrel for $79.05. Or you can save yourself the hassle of a rebuild and order a complete stainless steel assembly for $134.97. The only specialized tool you will need is snap-ring pliers to remove the circlip holding the master cylinder together, and only if you’re rebuilding from parts, not a full kit. As always, we suggest having a good shop manual on hand in case you find yourself wondering about some small detail.
Start by removing the brake lever. There is a 10mm nut below, and a Phillips drive screw above. Next, disconnect the brake hose at the master cylinder. Using a Phillips head screwdriver, remove the four screws holding the brake lever and master cylinder assembly to the handlebar.
Remove the small Phillips screw holding the brake switch components in the master cylinder half of the switchgear. If you didn’t remove the brake lever first, this is more difficult than it needs to be.
Transfer the handle and master cylinder assembly to your workbench. The brake fluid reservoir is held tight by a single nut and washer, with an O-ring sealing it to the master cylinder barrel. Remove the reservoir.
The gray barrel is the actual master cylinder. It’s screwed into the handle and locked in place with a small Allen head grubscrew. Locate the grubscrew. Using a 3/32-inch Allen wrench, remove the grubscrew.
Here’s the grubscrew removed. Once the grubscrew is out, unscrew the barrel from the handle assembly. It can take a little force to release. If necessary, lock the handle in a bench vise, being careful not to damage it.
Next, with the master cylinder barrel removed from the handle assembly, remove the snap-ring retaining the piston inside the barrel.
Often, you can reuse the master cylinder barrel by cleaning and honing the bore to return it to serviceable condition. This isn’t one of those times.
We knew we’d need new internal parts, and since the barrel was also beyond saving we sourced a new one made from stainless steel, which won’t corrode like the original steel barrel.
Since we rebuilt instead of replaced, we had to install each part individually. Here, we’ve just installed the new seal on the brake piston. Use brake fluid as a lubricant when installing the seal (and when assembling the rest of the master cylinder) and work slowly and carefully to avoid damaging the seal.
Assemble the barrel, replacing old parts with new in the same order as removed starting with the check valve, followed by the spring, the cup main seal, the spring washer and piston with new plunger seal. Make sure the seals are installed so they go into the barrel open-end first. Install the new snap-ring and finally the new rubber dust boot.
Now the tricky part. Screw the barrel into the handle assembly and replace the brake lever. With the lever fully applied, slowly unscrew the handle while applying low pressure air to the outlet of the barrel. At the first sign of air leaking from the reservoir holes, stop. The piston is now in position to allow fluid to return to the reservoir when the brakes are released. If not backed out the proper amount, your brakes will apply but not release. If backed out too far, you won’t have effective brakes. Loosen the handle one more turn and install the grub screw so it engages the flat in the barrel.
A close look at the master cylinder barrel (shown here partially assembled) shows the feed and return ports for the brake fluid reservoir; the feed port for pressure is the larger port to the left of the reservoir mounting stud while the return port is the smaller one to its right.
With the cylinder barrel properly installed and the grub screw tightened, replace the reservoir using the new O-ring, lubricating the O-ring first with brake fluid.
Here’s the reservoir installed on our master cylinder. Make sure all parts are absolutely clean before assembling.
Assembled and ready to go, we opted to conduct a quick bench test to confirm we had good pressure before final installation, just in case we’d done something wrong.
The next to last thing to do was to install the master cylinder/handle assembly back on our Triumph, hooking up the switchgear and making sure the switches worked before installing the hydraulic brake line. Make sure to use new copper sealing washers on both sides of the brake line banjo fitting.
The last thing to do is to bleed the hydraulic system, flushing fresh brake fluid through the lines until it comes out clean and clear at the brake caliper bleeder screw. If the brake handle is soft, try tying it back overnight to purge the last bit of air out of the line.
