This was one of the last jobs I did. I had disassembled the original master cylinder when I did the handlebars and was planning on getting a rebuild kit. However inspecting the inside of the cylinder I found some scores. I think that these could probably be machined or buffed out but given the importance of the brakes and the fact that so much money has been spent already on this project it seemed like false economy to re-use the old one! So a brand new complete master cylinder assembly was ordered from Munich Motorcycles along with new stainless steel brake lines.
Installing these is pretty easy- just bolt everything together. I have re-used the brass splitter which is necessary for the twin disc setup of the R65LS as well as the original steel lines at the calipers. These cleaned up pretty well.
After installation it was time to prime and bleed the brakes. This job is a little fiddly- and it’s made harder by doing a complete rebuild, and by having the twin disc setup. Basically you have to get fresh DOT 4 brake fluid into the system and get any air bubbles out. This should result in a nice firm feel on the brake lever and strong actuation of the front brakes.
To do this I took the following steps:
- Bike on centre stand
- Some clear vinyl tubing running from the bleed nipples into a container of brake fluid ( I used an old bottle of brake fluid that I wasn’t going to use again)
- Fill the reservoir on the handlebar with fresh DOT 4 fluid and loosely replace the reservoir cover.
- Unscrew the bleed nipples about a turn or so and pump (multiple pumps) the brake lever until fluid starts to come out of the nipples.
- At this stage the brake lines are somewhat primed and I commenced the standard bleeding procedure:
- Close the bleed nipples and pump the brake lever until resistance is felt. Then, holding the lever down, release one of the bleed nipples a quarter turn and feel the brake lever press down to the bar grip. Simultaneously observe the fluid coming out of the caliper into the tubing. You will see small bubbles. Repeat this process alternating sides until there are no bubbles.
- If all the lines have been changed there will be some pesky bubbles and your brake lever will feel spongy. Tapping along the lines and on the calipers with a screwdriver handle or similar, helps dislodge the bubbles.
- During this procedure, especially if doing a total system replacement you will go through at least 2-3 reservoirs worth of fluid. It’s important to keep an eye on the level and don’t let it drop too low. If the reservoir becomes empty you will start sucking air into the system from the master cylinder and then you’ll have to effectively start again.
- Also be careful with brake fluid as it is a pretty effective paint stripper. Best to cover brake levers, tanks, etc with shop rags.
