BLEEDING TRIUMPH BRAKES
Later versions of this article will include a section on the front master cylinder as well.
Replacing and Bleeding the Triumph Rear Master Cylinder
By William Getty
In the many years I was running my British motorcycle shop one job I didn’t look forward to was repairing the Triumph rear master cylinder. After some time however we developed methods that greatly streamlined this operation.
1. Always replace the rubber brake hose no matter how good it looks. We have had a number of hard to bleed brakes turn out to be the hose flexing.
2. The caliper is often blamed for the brake sticking on but it is extremely rare for the caliper to be bad. If not leaking don’t fix it.
3. You need not remove the entire master-cylinder assembly but often can unscrew the cylinder portion from the mounting body. Replacing the mounting body with its stiff return spring can be a real chore and time intensive.
4. Brake fluid is very corrosive so be careful about getting it on painted surfaces. Always use a new bottle of DOT 3 fluid to avoid contaminated fluid. Use Dot 3 as the components were designed for that grade. Brake fluid absorbs water from the air so avoid having the system open for long, keep the reservoir cap on as much as possible.
5. An inexpensive vacuum bleeder is available from Amazon for under $20. Money well spent. Amazon: ( HTOMT 2 in 1 Brake Bleeder Kit )
Step 1: To replace the rear master cylinder without removing the entire assembly from the machine remove the right hand muffler. Remove the rear wheel assembly. This makes accessing the cylinder much easier.
Step 2: Unscrew the brake hose from the caliper metal hose. (1) Remove the stepped bolt holding the caliper to the swing arm and set the caliper aside. Remove the brake hose from the master cylinder (2) and dispose of. It is highly recommended to replace this hose! PN 60-7028 1976-
79 under disc caliper, 60-7233 over disc caliper 1979 D model and 1980 on.
Step 3: Remove the plastic saddle that feeds the Master cylinder by bending the lock tab and unscrewing the large nut.(1) The master cylinder carrier body was treated with Loctite from 1977 on so officially was not rebuildable although rebuild kits are available. (PN 99-7022) We recommend replacing the cylinder barrel portion with a new stainless steel unit. (PN 99-7027).(3) You will need to heat the aluminum housing that the cylinder is threaded into. Be careful here as it is possible that flammable oil or grease may be present. Be sure to clean the area as much as possible. I like a small Butane torch to heat the housing.
Step 4: There is a small Allen grub 3/32” screw on the bottom of the aluminum housing (4) that retains the cylinder from unthreading. These can be very tight so be careful removing it. If it resists apply more heat. Take care not to strip the internal hex of this grub screw.
Step 5: Once the set screw has been removed use a 5/8” open end wrench to unscrew the cylinder from the alloy housing. If it resists apply more heat.
Step 6: The old cylinder is right where the rear tire throws all manner of stuff so is often full of rust. To examine remove the rubber boot and then the
circlip that retains the piston. Often the piston is seized in the cylinder. This can be driven out by inserting a long nail through the hose end and driving the piston out. You will be replacing all the seals so discard the old ones. Clean the piston and cylinder bore. Minor pits may be removed with a 5/8” brake cylinder hone (Flexhone # BC5812) but deeper pitting means replacement.
Step 7: If the cylinder is not damaged then use rebuild kit (PN 99-7022) following the instructions for assembly with the kit. Use DOT 3 fluid. All seals need to be coated with DOT 3 before assembly, do not assemble dry and use no petroleum based lubricants. It can be a bit of effort to get the new seals
into the cylinder so be patient.
Step 8: Assemble the rebuilt or replacement master cylinder, threading it into the carrier. A little anti-seize compound on the threads is advised here. (Permatex 80078). Thread the master cylinder in until the piston in the cylinder touches the push rod connected to the brake pedal. By lightly pressing the brake pedal down you should be able to tell when the push rod touches the piston. You need to have one revolutions worth of clearance here. Thread the cylinder in until there is no clearance between the push rod and piston then loosen until the slot or flat on the cylinder lines up with the grub screw and you have detectable clearance by very gently moving the brake pedal and feeling it connect with the master cylinder piston. Tighten the set grub screw into the groove in the master cylinder. Be sure the brake pedal will move through its entire range of motion. Click here for the Triumph manual explanation.
Step 9: Thread the new rear brake hose into the master cylinder, don’t omit the copper sealing washer here. Make sure the fluid reservoir is clean, if not use brake clean to flush it out. Do not use contact cleaner or carburetor spray as these will damage the parts beyond use. Attach the fluid feed saddle to the top of the master-cylinder using a new O ring (PN 60-4374).
Step 10: Connect the brake hose to the caliper steel hose. Suspend the caliper with a cord so as to keep it from stretching the new brake hose. Put a piece of wood or other spacer between the brake pads. Refill the reservoir with DOT 3 from a new container. Gently work the brake pedal and watch for bubbles in the fluid reservoir. Keep this motion up while rotating the caliper below the level of the master cylinder to allow any air bubbles to find their way up the line into the reservoir. This should with a bit of time purge all the air from the system. You will know this has happened when you get a consistently hard brake pedal that remains constant with time. If it gets soft after a bit continue working the pedal until you see no more bubbles in the reservoir and the pedal remains strong. The caliper should be gripping the wooden spacer with the pedal depressed.
Step 11: If you don’t get a strong pedal then try slacking the bleed nipple on the caliper and using the brake bleeder turn the caliper so the bleed nipple is as high above the master cylinder as the hose will allow keeping the nipple at the highest point. Connect the vacuum side of the Brake-bleeder to the bleed nipple and open the nipple by unscrewing 2 turns. Working the Brake-bleeder keep a close eye on the fluid level in the reservoir as this will draw a large volume of fluid through the system. After a few pumps of the bleeder close the bleed nipple and pump the brake pedal. You should have a consistently hard pedal that doesn’t get softer with time. Repeat as needed. If you do not get a hard pedal there is still air in the caliper.
Try rotating while using the bleeder to dislodge any hidden air bubbles in the system.
Step 12: Reassemble the stepped torque arm bolt to the caliper and swing arm. Remove the wooden spacer from the caliper taking care not to work
the brake pedal with this removed. Replace the rear wheel and caliper checking that it rotates freely. Work the brake checking operation.
Check the reservoir fluid level and adjust as needed to have it about half way full. Replace the cap.
