I’m converting this RS model to a RT. The only difference in the brake system is the length of the hose from the front master cylinder to the front brake line splitter underneath the gas tank as the RT handlebars are wider so that hose is longer. The rear brake system is the same on both the RS and RT models

I previously rebuilt the two front and one rear calipers and rebuilt the rear master cylinder. The front master cylinder was so badly corroded that I replaced it along with the entire right handlebar perch. I also installed the three disk brake rotors on the newly powder coated wheels. You can read about how I did that work here.

• 34 BMW 1983 R100RS Disassemble, Inspect & Rebuild Brembo Brake Calipers
• 34 BMW 1983 R100RS Disassemble, Inspect & Rebuild Master Cylinders
• 34 BMW 1983 R100RS Remove, Refinish, Install Disk Brake Rotors

Here is how I install the front brake system.

• 34 BMW 1983 R100RS/RT Install Front Brake System

Parts

Here is a list of brake system parts I replaced in the rear brake system not including the parts I replaced when I rebuilt the rear caliper and master cylinder. I found a cracked hose so I replaced that. The brake fluid reservoir was not damaged, so I cleaned it up but the hose from the reservoir to the top of the master cylinder was damaged so I replaced it.

Part #	Description	Qty
34 32 1 242 759	BRAKE HOSE – L=258MM, Rear to Caliper	1
16 12 1 116 458	HOSE CLAMP, Rear MC Fluid Line	2
34 32 1 236 259	HOSE, Rear MC to Reservoir	1
61 13 1 362 825	CLAMP – D=10MM, Rear Brake Hose	1

Tools

I use an “Air Zapper” suction pump to pull brake fluid from the brake fluid reservoir through the bleed hole in the caliper. This tool does a very good job removing air from the system.

I use a special tool to crimp the “Use Once” hose clamps that secure the hose to the brake fluid reservoir and master cylinder. I got the hose clamp crimping tool from Euro MotoElectrics

Video

Here is a video summary of how I did this work.

VIDEO: 1983 BMW R100RS/RT Install Rear Brake System

Install Brake Lines

NOTE:
The battery box has to be removed to install the steel brake line as it fits under the box. I already installed mine so I could test the electrical system. It’s easy to remove the battery and the box.

There is a steel brake line that screws into the master cylinder on the right side of the swing arm and routes to the left side of the swing arm where it fits into a bracket and attaches to the flexible hose that screws into the rear brake caliper.

The left side brake pipe bracket has a threaded hole. It mounts under the gusset that is between the rear frame tube and muffler bracket.

The bracket has a rubber grommet that secures the brake hose. It’s a very tight fit so I install the hose in the grommet before I mount the bracket on the gusset. I found if I clamp the hose in the soft jaws of my vice, lubricate the inside of the grommet with tire lube (or dish soap) and push very hard while rocking the bracket I can get the end of the hose to slide through the grommet.

I route the rear brake line under the battery box. I rotate the “J” end of the line that goes on the master cylinder to get it under the swing arm and then the rotate 90 degrees to align the other end with the brake hose bracket. I leave the bracket bolt loose so I can align the threads of the brake hose with the nipple on the steel line. After I get the fitting threaded tight, I tighten the bracket bolt.

I secure the rear steel brake line with two zip ties on each side of the battery bracket.

Mount Rear Master Cylinder Assembly

The rear disk brake has it’s own master cylinder. I had the rear brake master cylinder bracket powder coated and repainted the master cylinder after I rebuilt it.

Mount Master Cylinder To Bracket

NOTE:
The following sections contain pictures I took on the workbench so I can show the details more clearly. There is not much room to take pictures where the rear master cylinder and bracket are installed.

The master cylinder mounts to a bracket on the right side of the swing arm with two Allen bolts with wave washers that screw into tapped holes in the rear of the bracket.

Attach Brake Foot Pedal Linkage

The master cylinder connects to the rear brake pedal with a linkage and a clevis on each end. Each clevis uses a special pivot pin that snaps onto the shoulder of the clevis.

The Pivot Pin Slides Into The Back Of The Clevis.

Then the pin is rotated until the collar snaps over the shoulder of the clevis.

The end of the return spring on the mechanism fits into a hole in the muffler bracket. The return spring hooks into a small hole on the tab that fits on the threaded linkage. It is retained with a nut. The end of the tab the spring fits into should be on the side so the spring is on the outside of the master cylinder (not as shown in the picture below). And the hook on the other end of the spring (covered by my fingers in the picture below) should point to the outside as well so it will fit into the hole in the muffler bracket.

NOTE:
It is possible to install the spring on the tab in two positions. Make sure it’s installed so the hook on the other end of the spring points toward the outside.

Mount Master Cylinder Bracket To Swing Arm

The front end of the bracket attaches to a tab on the swing arm with and Allen bolt that threads into a tapped hole in the tab and the rear end attaches on top of the muffler bracket using the muffler bracket Allen bolts to secure it.

I align the front slot in the bracket with the threaded hole in the bracket on the bottom of the swing arm making sure the rear plate of the master cylinder bracket is under the bottom of the muffler bracket. I attach the front bolt with the hardware loosely.

I install the rear brake pedal with it’s hardware. With the rear bracket loose, I can align the threads in the rear of the master cylinder with the fitting on the end of the steel brake line. I tighten the fitting “good and tight”.

NOTE:
This bike has non-standard brake pedal mounting hardware, so I’m not showing it in detail.  The hardware works so I’m keeping it. 

I pull back the brake pedal return spring and put the hook through the hole on the rear of the muffler bracket.

Install Rear Fluid Reservoir

NOTE:
Some of these pictures were shot on the workbench so you can see the detail more clearly.

The rear brake fluid reservoir hose attaches to the reservoir and to the nipple on the top of the master cylinder with “use once” crimped hose clamps. I will crimp the clamp on the master cylinder when I get it mounted on the master cylinder bracket.

NOTE:
I crimped the hose clamp with the crimp pointing to the outside as shown in the picture below. But, it is better if you place the crimp 180 degrees from where I put so it points toward the inside of the master cylinder to create more clearance for the brake pedal return spring when you mount the master cylinder.

After installing the steel brake line I mounted the battery box. I attach the rear brake fluid reservoir to the long stud of the vibration isolation stud on the top, right side of the battery box using the battery box hex nut and wave washer. I rotate the reservoir so the “MAX”, “MIN” lettering is facing to the outside and then I tighten the Phillips screw that secures the reservoir in the bracket.

I verify that the steel brake line and the reservoir fluid hose don’t interfere with each other. I also make sure the rear brake pedal return spring doesn’t catch on the rear passenger peg nut or on the brake line. It’s a tight quarters.

Install Rear Brake Caliper

The rear brake caliper mounts on a caliper plate that is secured by the rear axle. The caliper plate is kept from rotating by a brake stay that mounts in a bracket on the bottom left side of the swing arm and a hole in the bottom of the caliper bracket.

Torque Bolts Securing Caliper Halves Together

I did not torque the caliper mating bolts which is why there is red tape on them. I thought it would be easier to torque them when the caliper was mounted, but there isn’t a lot of room so I clamp the caliper in the soft jaws of my vice and torque them to 22 FT-Lbs.

The rear brake caliper mounts on the opposite side (left) from the master cylinder (right). It is secured to a plate the axle goes through with two bolts that screw into threaded holes in the plate. The rear bolt is shorter than the front bolt.

Bleed Rear Brake System

But, before I mount the caliper on plate, I bleed the rear brake system. It’s easier to bleed all the air out of the system if I hang the caliper on the rear shock with some wire so the brake hose goes up to the caliper, but the caliper is still below the bottom of the fluid reservoir on the right side. I want air bubbles to rise up the hose so they can easily exit the bleed valve on the caliper which they do in this orientation since air bubbles are lighter than the brake fluid.

NOTE:
In the picture below, I have the hose and bleed valve in the wrong holes. They should be swapped. I figured that out when I went to mount the caliper and saw the hose would have a sharp kink in it. So I had to open up the system to air again to swap the hose and bleed valve and then bleed the air out again. But it didn’t take long to get a solid rear brake.

I insert some wood shims tightly between the brake pads so when I pump the rear brake pedal the pads and the pistons behind them don’t move out of the caliper.

I fill the rear brake fluid reservoir with fluid from a brand new bottle of DOT 4 brake fluid. I make sure the bleed valve on the caliper is closed. Then I pump the rear brake pedal repeatedly and watch for bubbles coming up into the reservoir from the hole the fluid line attaches to. Eventually all the air in the caliper will exit via the hole and at that point the master cylinder is filled with fluid. I also watch for a ripple in the surface of the fluid as I release the pedal indicating that the fluid in the master cylinder is returning to the reservoir via the small return hole in the master cylinder. That confirms the hole is not blocked and the master cylinder is working correctly.

Next I attach the Air Zapper to the bleed valve on the caliper which is finger tight. I make sure the rear brake fluid reservoir is full and turn on the Air Zapper. Then I open the bleed valve a small amount. Air bubbles and brake fluid exit the bleed valve down the hose into the container. I keep an eye on the fluid level in the reservoir and keep sucking brake fluid out of the system until there don’t appear to be any air bubbles in the stream. Then I close the bleed valve and turn off the Air Zapper.

There is still some air in the brake system. I now use the traditional method of bleeding brakes. I push the rear brake pedal all the way down, open the bleed valve to let fluid and any small bubbles exit, close the bleed valve and then release the brake pedal.

I have found that pumping the rear pedal four or five times with the bleed valve closed helps push the air down the lines to the caliper. Then I push the pedal down again and open and close the bleed valve. In a couple of minutes the pedal is getting hard at the end of its stroke and I can see the brakes pads clamping the wood shims. This means I have all the air our of the system.

Mount Rear Caliper To Caliper Mounting Plate

Before I mount the caliper on the caliper mounting plate, I use brake cleaner on a clean blue shop towel and wipe down the disk on both sides until there is no sign of dirt or oil on the towel.

The caliper mounts with two bolts into tapped holes in the caliper plate. The bolts are not the same length. I put the long bolt in the front hole and the short bolt in the rear hole. I rotate the caliper mounting plate on the axle so the holes for the caliper are where I can get a wrench on them. I finger tighten the bolts and then torque them to 20 FT-Lbs.

Install Brake Stay On Caliper Plate

I rotate the caliper plate so the hole the brake stay fits into is toward the bottom. I install the end of the stay with the small Allen bolt and lock nut between the legs of the bracket on the bottom of the swing arm. I insert the Allen bolt and finger tighten the lock nut.

I adjust the caliper plate so I can insert the larger bolt and secure it on the back of the plate with special serrated flat washer and the lock nut.

Install Rear Brake Hose Retaining Clip

Next I install the rear brake hose retaining clip on the pin welded on the inside of the left swing arm leg and clip it onto the brake hose to help keep it out of the way of the wheel.

Final Testing

I rest the brakes now that the caliper is installed. The pedal is firm and it stops the rear wheel. I adjust the right front foot peg so my foot is just above the pedal. Then I test the rear brake light works when I press on the pedal.  All is good so the rear brake system is installed and working.

Revisions

2020-08-11  Add link to Air Zapper.
2020-08-14  Add link to installing front brake system.
2020-08-17  Add install of rear brake hose retaining clip.