This bike had the blue front calipers and the ATE front disk brakes with the drum rear leading and trailing shoe brakes. Originally, the wheels were wire spoke but a previous owner upgraded them to the later snowflake wheels.

I rebuilt the two front calipers, the master cylinder and replaced the rear brake shoes. The blue anodized calipers had faded so I had them anodized, but this was a dismal failure. Either the shop who did the work botched it or it’s not so easy to anodize old parts. I found a paint that replicates blue anodizing. I stripped the botched anodizing and painted them. I like the result.

I replaced the lower steel brake line that goes between the caliper and the union that is inserted into the lower front fairing with stainless steel lines from Rocky Point Cycle. I replaced the rubber lines with braided steel lines with a translucent blue cover and chrome unions from Spiegler Performance Parts.

Tools

A set of snap ring pliers are used to disassemble the front master cylinder. Other than that, typical wrenches, sockets and screw drivers are used.

Parts

Here is a list of the parts I used. I replaced the steel brake lines from the calipers to the rubber brake line with stainless steel lines from Rocky Point Cycle. I replaced the rubber brake lines with braided steel lines from Speigler. I chose chrome fittings and translucent blue covering to look similar to the blue of the anodized calipers. The original rubber lines are black, but I’m not opposed to adding small touches that personalize the build.

Part #	Description	Qty
34 11 2 301 7091	D40 ATE REPL SEAL KIT CALIPER	2
34 31 1 234 927	SET: REPAIR KIT BRAKE MASTER CYLINDER – D=17MM (to 09/80)	1
ATE 9361	Rocky Point Cycle: BRAKE PIPE LEFT	1
ATE 9363	Rocky Point Cycle: BRAKE PIPE RIGHT	1
34 32 1 234 629	GROMMET (to 09/80)	2
34 11 1 233 124	VENT SCREW (to 09/80)	2
S-BM0085	Spiegler: Braided Steel Brake Lines-Translucent Blue / Chrome Fitting	1
34 11 1 231 469	O-RING (to 09/80)	2
35 21 1 230 360	GASKET RING (to 09/80)	1
34 11 2 301 358	SET: REPAIR KIT, BRAKE PADS – TEXTAR T290 (to 09/80)	2

Remove Master Cylinder and Brake Lines

The procedures for the 1977 R100RS are the same as those for the 1975 R75/6 project with the exception of the brake lines and the master cylinder. The R75/6 has a single front disk while the R100RS has two. You can read about the procedure here.

• 34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper
  • Remove Master Cylinder

I remove the gas tank to access the master cylinder mounted on the spine tube.

I open the bleed screws on the calipers and pump the front brake lever to flush the old brake fluid out of the master cylinder.

I use a clean blue shop towel to mop up the last bit of brake fluid out of the master cylinder reservoir. I remove the front brake cable from the master cylinder by loosening the large screwed ferrule to slacken the brake cable far enough to remove the barrel from the fingers of the master cylinder lever.

Then I unscrew the two steel brake lines from the master cylinder body and remove the hose clamp that secures the master cylinder to the spine tube.

These go to fittings in the upper center fairing panel where they connect to rubber hoses going to each caliper. I unscrew the steel lines from the rubber hose fitting.

The other end of the rubber house goes to the steel lines mounted on the lower fork slider that attach to each brake caliper. I remove the rubber hose from the steel caliper line at the chrome bracket at the top of the fork slider and then remove the steel line from the caliper body.

Remove Calipers

Removing the calipers on the 1977 R100RS is the same procedure as I used on the 1975 R75/6.

• 34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper
  • Disassemble Caliper

The calipers attach to the lower fork sliders with an eccentric pin that is underneath a cap nut that screws into the bottom of the plate the caliper swivels within. I unscrew the cap nut and remove the spring between it and the bottom of the eccentric pin. I use an 8 mm bolt to catch the first few internal threads of the eccentric pin to pull it out. The bolt will not screw all the way in and I don’t force it.

The caliper is now free and I remove it from between the mounting plates on the lower fork slider. The pads are rusty and in need of replacement.

The fixed brake pad is secured by a “hairpin” retaining clip on the back side of the caliper.

The other pad faces the piston inside the caliper and has a center pin with a small o-ring on it. I pull it out of the piston.

I use compressed air to push the piston out of the caliper cylinder into a towel to protect it. These pictures are from the work I did on the 1975 R75/6.

The piston is not rusty but the dust seal tore when the piston came out. Note the small rubber o-ring that fits on the center pin of the movable pad.

I use a pick to remove the body of the seal that has a metal ring.

Then I use the pick to pull out the rubber piston seal inside the caliper bore.

Here are the caliper parts.

You can see how faded the caliper front is. The calipers were anodized using a blue dye. After 40 years it’s mostly gone.

I’m going to have the calipers anodized again so I remove the plastic buttons on the top and bottom of the caliper. These let the caliper swing on the eccentric pin between the top and bottom plates of the lower fork slider. I use a small blade screw driver to gently wiggle the button out of the hole.

Black Plastic Caliper Button Dimensions

The plastic buttons are no longer available. I took some pictures and measurements so you can make a replacement should you loose one.

Refinish Front Brake Calipers & Master Cylinder

I took the calipers to a local shop that does aluminum anodizing. I was told they would strip them and anodize them with blue dye. There is a minimum fee for stripping and anodizing so a friend with a 1977 R100RS with faded blue calipers sent them to me so we split the cost.

To make a long story short, the new anodizing was a disaster. The color was uneven and every one of the four calipers was a different shade of blue. The surface was rough and I could remove the anodizing by rubbing my fingers across it.

I used a wire wheel to remove all the new anodizing. I used “000” steel wool and polished the caliper so it was shiny. Dupli-Color makes a blue anodizing paint that I use to refinish the calipers. It fully cures in seven days. I sprayed some on a piece of scrape aluminum, waited seven days and then applied brake fluid and rubbed. I got only a faint smudge of blue to come off, so this is very resistant to brake fluid.

I bead blasted the master cylinder to remove the rust and old paint. I taped off the brake fluid hole on top, the piston hole, the arm, the brake line holes and the brake light switch hole I used engine paint and painted it black, waited seven days for it to cure and then baked it at 200 F for an hour to harden the paint. The paint is quite resistant to brake fluid when prepared this way.

Rebuild Master Cylinder

The procedure to rebuild the 1977 R100RS master cylinder is the same as for the 1975 R75/6. You will find the procedure here.

• 34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper
  • Disassemble Master Cylinder Piston
  • Clean Up Master Cylinder Bore
  • Install New Parts In Master Cylinder

When I removed the master cylinder, I checked the bore the piston sits in for signs of rust. This is common if the brake fluid is not changed as it absorbs water which will collect in the master cylinder bore and rust it. It was clean. I used a small socket with 600 grit wet/dry paper taped to it and honed the bore so it shines. I flushed it with brake cleaner and ran a blue shop towel into it to be sure the towel came out clean with not grit, metal flakes or discoloration.

Here are all the parts that are included in the master cylinder.

Starting from the left is the brake cable nipple holder, the pin that goes between the actuating arm on the master cylinder and the rear of the master cylinder piston, the master cylinder body with the gold color actuating arm, the return spring, the piston with seals, the c-clip to retain the piston in the master cylinder, the plastic brake fluid reservoir and the threaded bolt that secures the reservoir to the top of the master cylinder.

I cleaned up the front brake light switch. I used copper cleaner to remove the corrosion on the contacts and also some fine steel wool on the contacts and the switch body. I used a wire wheel to clean up the treads. Note that this switch uses a pipe thread and does not need the aluminum washer shown in the parts fiche.

I tested the front brake light switch as shown in this short video. The switch is working so no need to replace it.

There are two holes in the top of the master cylinder where the plastic reservoir mounts. The larger one lets fluid into the piston to pressure the brakes and the smaller one lets fluid back into the reservoir when the return spring pushes the piston back. I check the smaller one to be sure it is open and not obstructed using a strand of wire from a piece of stranded copper wire.

There is a new o-ring in the kit that seals the fluid reservoir to the master cylinder body.

Refer to the above links about rebuilding the master cylinder on a 1975 R75/6 to see the procedure I used on the 1977 R100RS.

One thing I noted is a difference between the original master cylinder piston and the replacement. The front of the original piston has three holes while the replacement has six.

Here is the new master cylinder piston with seals that is supplied in the rebuild kit. I soaked the seals in DOT 3 brake fluid before putting the piston into the master cylinder bore so they would slide in easily.

After I inserted the spring into the bore, it took a couple attempts to get the piston to pass into the bore. I found it easier to install the c-clip by mounting the master cylinder in the rubber jaws of a vice, using a drift to push on the piston with one hand and then inserting the c-clip held in c-clip pliers into the groove with the other.

Then I screw in the front brake light switch. Here is the repainted and rebuild master cylinder. The pin that goes between the actuating arm and the center of the piston is on the right.

Rebuild Front Brake Calipers

The procedure for the 1977 R100RS is the same as for the 1975 R75/6 except there are two calipers to rebuild.

• 34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper
  • Brake Caliper Rebuild

NOTE:
The 1975 R75/6 has 38 mm diameter caliper pistons while the 1977 R100RS has 40 mm pistons. The 38 mm caliper has no marks on the outside of the caliper while the 40 mm ones have “40” inscribed into them.

There is no left or right caliper as the same caliper will fit on either side.

I cleaned the piston and the bore with DOT 3 brake fluid to be sure there was not grit or dirt on them.

The caliper rebuild kit has two parts, the inner rubber piston seal and the outer dust cover that fits on the end of the piston. I also got new bleed nipples.

Due to the failed attempt to anodize the calipers, I used 1000 grit wet/dry and polished the bore inside the caliper to be sure none of the badly done anodizing would flake of.

I made sure the dust seal was evenly embedded in the caliper body.

Install New Brake Pads

The procedure for the 1977 R100RS is the same as for the 1975 R75/6. You can read how I did it here.

• 34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper
  • Install Brake Pads in Caliper

The new brake pad comes with a new rubber o-ring for the movable pad and the “hairpin” spring to secure the fixed pad. The fixed pad has two buttons on the back (the bottom one with the white BMW logo on it in the picture below) that fit into the circular cut out on the back of the caliper.

Here is a caliper with the new pads installed.

I install the original plastic buttons in the top and bottom holes of each caliper.

Install Front Brakes & Master Cylinder

The procedure for installing the calipers in the 1977 R100RS is the same as for the 1975 R75/6. You can read how to do it here.

• 34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper
  • Install Caliper in Front Fork

Here are the refinished and rebuilt calipers installed on the fork sliders.

The new stainless brake lines I got from Rocky Point Cycle are different for the right and left side. To install them, I start by screwing the line into the lower port of the caliper.

I remove the chrome brake line bracket from the lower fork slider and then insert the other end of the stainless steel line through the rubber grommet in the bracket.

I had to bend the line a bit to get it align with the hole in the chrome bracket. Then I installed the bracket with the end of the steel line in the grommet.

I screw the fitting on the end of the Speigler braided steel brake hose to the end of the Rocky Point Cycle stainless steel line. I orient the hose so the bend is on the bottom so it curves upward toward where it will mount in the upper center fairing panel.

I attach the master cylinder to the spine tube with the large hose clamp so the brake light switch is next to the steering head.

Here is how the line is oriented. I put a piece of tape on the end of the line that goes into the master cylinder when I removed the line so I would not have to figure out which end connects to the master cylinder.

Then I screw the end of the upper steel brake lines into each side of the master cylinder.

Here is how they route from the master cylinder through the front of the frame. The right one is very close to the steering damper. I set the damper to “2” which moves it closest to the brake line to be sure it doesn’t rub on the line. I had to bend it a bit to get good clearance.

I will wait until I install the top center fairing panel to connect the upper steel lines to the Spiegler braided steel brake hoses which fit in a grommet inserted in the fairing panel. I’ll likely have to adjust where the master cylinder is mounted to get the steel lines to fit in the rubber grommet in the fairing.