This article was originally published on thebonnevilleshop.com 

Servicing The Hydraulic Disc Brake
Service Bulletin 4/72

By David Porter, Customer and Technical Support at The Bonneville Shop

Service Bulletin 4/72 for Triumph Hydraulic Disc Brake was published 10/24/72, around the time that the Lockheed hydraulic disc brakes debuted on the new Triumph 750cc models: TR7RV, T140V, and T150V. This bulletin seems to have preceded the 1973 Triumph Factory Workshop Manual, 99-0983, and was of significant importance, as the new 1973 models were starting to appear in dealer’s showrooms in the fall of 1972.

Hydraulic brake systems were relatively new to the British motorcycle industry during this time period, and it was critical that dealership service departments had an understanding of how to correctly service the new system. According to the Uncrating and Servicing Instructions found in Service Bulletin 5/72, there was no initial requirement to attend to the front brake system, as they were factory sealed and tested prior to delivery. However, proficiency with the new brakes was very soon to be a necessary skill in the wheelhouse of the Triumph service technician.

The actual process of removing air from the hydraulic brake system is well documented in the bulletin and is a very familiar practice to many of us that own or have serviced Triumph and Norton motorcycles from the 1970s. Extreme care has to be taken to keep any DOT3 or DOT4 brake fluids off of painted surfaces, as it will dull or remove paint. The bleeding process is straightforward on the front brake system, but it gets really fun when the rear hydraulic system needs attention. The trick here is to withdraw the rear wheel spindle far enough to allow the torque arm and brake caliper to be lowered, so the bleed nipple is facing upwards. This helps prevent the air bubbles from being trapped in the brake line. Be sure to remember to stuff a wrench or other suitable spacer between the brake pads when bleeding the rear caliper, or the brake pads will lock together and will have to be pried apart, which can damage the friction material of the pads.

Brake hoses and pipes should be inspected and replaced if there is even a shadow of doubt about their integrity, and sealing washers should always be replaced when updating hoses. Brake pads are easy to inspect and replace, as they are retained by a pair of large split pins. Remember the fluid level in the reservoir cup will rise when the pistons in the caliper are pushed into their bores during the brake pad replacement, so keep a rag handy in case of any spillage of the brake fluid.

One final word about brake system bleeding and flushing that is worthy of mention, is the use of a vacuum-operated brake bleeder, or “Vacula”. This tool is operated using compressed air, and can help quicken the bleeding process, but is really helpful when performing a brake system flush. (Get on  here: Deluxe Vacuum Tester Kit PN# TBS-35-5900)

This process takes a little practice to get accustomed to using the Vacula but is very effective in flushing impurities from the hydraulic system quickly. The one thing that is critical is to always maintain a fluid level in the reservoir. If you allow the reservoir to run dry while flushing the brake system, it will introduce air into the system, and you will have to start the bleeding process all over again. I keep a bottle of DOT4 fluid close by and keep filling the reservoir with brake fluid while the Vacula is locked on and pulling fluid through the system with the bleed screw open. Once I feel a firm brake lever (or pedal), with the bleed screw closed, I stop the Vacula and start pumping the lever to release any remaining microbubbles from the system. Top off the reservoir cup to the desired level of fluid and you’re done.