The 1983 R100RS has three Brembo disk brake calipers, two on the front and one on the rear. The front calipers have more stopping power than the original ATE disk brakes that were replaced by the Brembo disk brakes starting in the 1981 model year, in part because the front master cylinder is integrated into the right throttle assembly eliminating the cable, which  stretches, and goes between the brake lever and the under-the-tank ATE master cylinder, and in part because the Brembo calipers have two hydraulic pistons instead of only the one used in the ATE calipers.

I had to replace the front master cylinder due to corrosion but the rear brake master cylinder was in good condition so I rebuilt it. You can read about that work here.

• 34 BMW 1983 R100RS Disassemble, Inspect & Rebuild Master Cylinders

I removed the brake system when I disassembled the bike down to the frame. You can see how I removed the calipers here.

• 34 BMW 1983 R100RS Remove Brake System

Parts

Part #	Description	Qty
34 21 1 238 059	SET: MOUNTING PARTS – BREMBO (from 09/80)	3
34 21 1 236 793	VENT SCREW-Calipers, w/ Dust Cap	3
34 21 1 237 234	SET: REPAIR BRAKE CALIPER – D=38MM (from 09/80)	3

Video

I made a short video showing the master cylinder disassembly, inspection and rebuild procedure.

Disassemble Brake Caliper

The Brembo caliper consists of two castings that are bolted together with 10 mm bolts. I start with the rear caliper.

Remove Brake Pad Cover

In the center of the caliper, there is a plastic cover that I pop off with a small blade screw driver. Underneath the cover is a bracket assembly secured with two pins that go through both halves of the caliper to hold the brake pads against the pistons.

Remove Pad Retaining Clip Assembly & Pads

I use a drift to drive the pins out of the caliper so I can remove the bracket assembly and the pads.

NOTE:
It’s easier if you use a vice with soft jaws to hold the caliper while you drive the pins out with the drift.

The bracket assembly has a bracket with a loop on one end that the pin closest to the brake line and bleed valve goes through. The other end of the bracket has a half loop that fits under the other pin. There is a short pin that does not mount in the caliper halves that fits under the middle of the bracket and on top of the pads to keep them in the proper position inside the caliper.

Remove Caliper Clamping Bolts

I put the caliper in the soft jaws of my vice so I can remove the two 10 mm bolts. One of them is very rusty. The bolts are marked “8.8” which is the strength of the bolts.

When I get the two bolts out, the halves separate exposing a small o-ring the fits around a brake fluid transfer hole that is machined into each half on the end the brake pipe threads into. Since there is a puck in each half, the brake fluid needs a passage between the halves and the rubber o-ring seals that passage so it won’t leak.

NOTE:
I use the term “piston” and “puck” interchangeably for the piston inside the caliper the pushes the brake pad against the rotor.

Remove Dust Cover

I remove the dust cover from both pistons using a small screw driver to get underneath it.

Remove Piston

I use compressed air to blow the puck out of the caliper body. I loosely wrap the caliper with a rag and place it on a bed of rags so when the puck comes out it won’t be damaged or smack into the work bench and potentially my face.

I put my finger over the brake fluid transfer hole and after a couple short blasts of air, the piston comes out of the bore.

NOTE:
You may want to wear a mechanics glove when you put your finger over the hole as the air can blow past your finger and bruise it. DAMHIK. 🙂

Remove Square O-ring From Caliper

I remove the square o-ring inside the caliper with a pick. I’m careful not to scratch the groove when I put the pick under the bottom of the o-ring and twist it upward which pushes the seal out of the groove.

Inspect Calipers and Pistons & Clean Them

I inspect the piston and the bore for corrosion, pits and scratches. They look fine.

Inside the bore is a casting mark that shows the year and month the caliper was cast, (April 1982).

When I disassembled the front calipers, they were full of gunk due to old brake fluid. There condition matches what I found when I disassembled the front master cylinder. But, the caliper and pucks were not scratched or rusted so they are reusable.

I use brake cleaner to clean the caliper bores and the pistons.

WARNING:
It’s safer to NEVER use anything but brake fluid or brake cleaner on the inside of the caliper and the pucks. Solvents can damage the new seals, so play it safe.

I did have to polish the inside of the caliper using a brash brush on Dremel tool as low speed as a bit at the bottom to remove some rust deposits from the master cylinder. I was very careful not to scratch the bore of the caliper. Since the calipers are cast from an aluminum alloy, and the pucks are cast from a phenolic, they didn’t rust.

Paint Calipers

I paint the calipers with Duli-Color brake caliper paint. I use a metallic blue anodize color on the front calipers to hark back to the first year RS model in 1977 that had blue anodized ATE calipers as a tribute to the genesis of the RS/RT models.  I paint the rear black.

Before I paint the calipers, I mark the inside face to distinguish the front right, left and rear caliper halves so they will go back together with their mate. Then I mask off the inside mating surface and the caliper bore. I don’t want any paint on those surfaces. The mating surfaces have to be dead flat for a tight fluid seal of the transfer holes or the calipers will leak.

The caliper paint requires seven days to harden completely. After waiting seven days, I bake them at 220 F for 20 mins before rebuilding them. The caliper paint is resistant to the brake fluid, but it’s a good idea to let it cure completely before exposing it the fluid.

Rebuild Caliper

The Brembo caliper rebuild kit comes with instructions starting with removing them from the bike through installing them back on the bike. The instructions are in eight different languages and are printed on a very large sheet of paper. They mix different versions of the calipers together so I found them a bit hard to follow.

The parts include two dust seals, two square O-rings, two 10 mm caliper clamping bolts, the small o-ring that seals the brake fluid transfer holes in the caliper halves and grease to lubricate the square O-ring and the circumference of the piston to help install the piston.

Install Square O-ring

I put the square O-rings in some DOT 4 brake fluid for a few minutes to “season” them.

I install the square O-ring into the groove of the caliper half and work it around the groove until it is seated flush.

Install Piston

I put a coating of grease on the installed square O-ring and the walls of the bore in the caliper. I also put some on the circumference of the piston.

NOTE:
I put a “B” inside the puck that came out of the caliper half with “Brembo” cast into it so I would install the pucks into the caliper half and left or right caliper it came from. Since the puck and caliper have been getting along together and have mated for 36 years and 84,000 miles, it would be a shame if they got divorced now 🙂

I align the piston squarely in the bore and then push down with two fingers to insert it into the caliper.  If it won’t go in that easily, you don’t have the piston squarely aligned with the bore. Pull it back out (you many need to use some pliers on the lip of the piston to pull it out), and try again. You do not want to pound it into the caliper as you will damage it and/or the puck.

Then I repeat this for the other caliper half.

Install The Dust Seal

The dust seal stretches over the lip of the piston. It’s easy to install.

Ensure Caliper Inside Mating Surfaces Are Flat

It is important that the mating surfaces, the flat areas I didn’t paint at the top of the calipers, are dead flat. The brake fluid transfer holes can leak if this isn’t the case.

When the 10 mm bolts are torqued, they can pull up the edges of the holes and that will interfere with complete sealing of the brake fluid transfer holes.

I use a thick piece of coffee table glass for a flat surface. I tape a piece of 600 wet/dry sand paper to the glass, put a little water on the paper, and then lightly sand the flat surfaces in a figure-eight pattern.

I had marked the halves with “R”, “L” and “RR” to identify the right and left front caliper halves and the rear caliper halves. I continued the sanding until these paint markings are removed.

As you can see, some of the original black anodized finish on the flat mating surface got removed around the bolt holes and at the edges of the flat matting surfaces, particularly on the caliper half that has the threads, so it was worth doing this as the edges of the holes were raised.

Assembly The Caliper Halves

I put a dab of grease on the small O-ring and install it in the groove in one of the caliper halves. Then I thread in the two 10 mm bolts and snug them up.

I don’t want to risk damaging the new caliper paint so I will torque the bolts to 20-22 Ft-Lbs after I mount them on the fork legs. In a picture at the end of the next section, you will see some red tape on the calipers that I use to remind me that the clamping bolts need to be torqued.

Install Brake Pads, Retaining Clip Assembly, Pad Cover and Bleed Valve

I got a kit with new pins and the retaining clip along with new brake pads.

I use the brake grease to lubricate the long pins so the pads will move freely and the pin won’t corrode. I also put a dab on the edge of the puck to prevent corrosion between the puck and the back of the brake pad.

NOTE
This is not the same as the “brake grease” supplied by Brembo and you don’t want to use the Brembo grease here.

I make a sandwich of the two pads so the friction material doesn’t get an grease on them when I insert them into the caliper between the two greased pucks.

I insert the pin through the caliper half with “Brembo” on it into the hole closest to the holes for the brake pipe and bleed valve and through the slot in the first brake pad. Then I put the end of the pin through the circle end of the retaining clip, being sure the arch of the clip is facing the top of the caliper, and push the pin through the slot in the other brake pad and into the hole in the other half of the caliper.

I put a dab of brake grease on the short middle pin and then I place the short pin on top of the grooves in the top of the two brake pads. I fold the retaining clip over it. Then I push the second long pin through the other hole in the caliper and use a screw driver to push down on the other end of the retaining clip so the pin can slide through the half circle. I guide the pin through the second brake pad slot and into the hole in the other caliper half.

I use a drift to drive the pins into the caliper halves until they are tightly installed. A couple good raps will do it. I check the tapered end of the pins to be sure the pins are far enough through the hole on the back half of the caliper that some of the untappered portion of the pin is through the hole.

I verify the retaining clip is centered on the two long pins and the short middle pin. Then I install the black plastic brake pad cover.

I install the new bleed valve in the hole in the back half of the caliper and I put some red tape on the 10 mm clamping bolts to remind me they need to be torqued to 20-22 Ft-Lbs. It is easier to torque them after I mount the calipers to the fronts forks and the rear caliper bracket.