- Disassemble Front Master Cylinder & Inspect
- How To Install Front Master Cylinder Piston Assembly
- Disassemble Rear Master Cylinder & Inspect
- Polish Rear Master Cylinder Bore
- Assemble Rear Master Cylinder Rebuild Kit
- Install Piston and Assemble Rear Master Cylinder
- Replace Rear Master Cylinder Foot Brake Pedal Linkage
This bike has two master cylinders, one for the front brakes that is integrated into the right throttle assembly and the other for the rear brake mounted on the inside of the right side rear foot peg bracket above the swing arm. I show how to remove them here:
The brakes on this bike are made by Brembo and so is the rear master cylinder. However, the integrated front master cylinder is made by Magura who supplies the throttle assembly. There are master cylinder rebuild kits available for both the front and rear master cylinders.
I also rebuilt the three brake calipers and you can read about how I did that work here:
Each master cylinder has a different rebuild kit. Since my front master cylinder was rusted, I only purchased and installed the rear rebuild kit, but I list both master cylinder rebuild kits in the table below. Both kits are available from Euro Motoelectrics, where I got my rear master cylinder rebuild kit, as well as from your local BMW dealer.
To further complicate things, I managed to damage the throttle assembly denting the thin-walled tube the throttle slides inside of when I dropped it on the floor and inadvertently stepped on it. 🙁 My attempt to repair the dented housing broke the thin wall.
I decided to replace the throttle assembly, fluid reservoir and front master cylinder as this entire assembly costs just a bit more than buying the throttle assembly casting and new master cylinder assembly. So, the front master cylinder and throttle assembly should be good for another 36 years or so. 🙂
|REAR MASTER CYLINDER|
|34 21 1 242 791||SET: REPAIR KIT BRAKE MASTER CYLINDER (from 09/80), Rear||1|
|34 31 1 238 040||PROTECTION CAP, Rear MC||1|
|35 21 1 238 039||PLATE, Rear MC Linkage||1|
|07 11 9 987 821||FORKHEAD – G6X12-CD, Rear MC Linkeage||1|
|35 21 1 238 038||TENSION SPRING, Rear Brake Pedal||1|
|16 12 1 116 458||HOSE CLAMP, Rear MC Fluid Line||1|
|34 32 1 236 259||HOSE, Rear MC to Reservoir||1|
|35 21 1 236 231||PULL ROD, Rear MC Linkage||1|
|07 12 9 934 961||PIN – 6X12, Rear Brake Pedal Pin to MC||2|
|FRONT MASTER CYLINDER|
|32 72 2 302 372||BRAKE MASTER CYLINDER – D=14,29, Front MC Rebuild Kit *||1|
|32 72 2 302 368||HANDLE UNIT, RIGHT – D=15MM (from 09/81), Throttle Assembly w/ MC||1|
|34 32 1 242 205||HOLLOW BOLT (from 09/80), Front||1|
|07 11 9 963 072||GASKET RING – A10X13,5-CU (from 09/80), Front||2|
I didn’t buy the front MC rebuild kit as I replaced the entire throttle assembly (part# 32 72 2 302 368) which includes a complete front master cylinder assembly.
Disassemble Front Master Cylinder & Inspect
The front master cylinder has a 15 mm bore, as indicated on the body, and appears to be manufactured by Magura who supplies the left side throttle assembly.
There is a front dust seal and large flat washer that are installed in the throttle assembly. The seal fits over the end of the piston nose and the flat washer protects the seal. The brake lever presses on the end of the piston nose to pressurize the front brakes.
Front Master Cylinder Video
I made a short video showing how I disassemble it.
Remove Piston Assembly
The front master cylinder piston assembly is secured inside the cylinder with a C-clip. I use my C-clip pliers to remove it.
Inspection – FAIL 🙁
Inside is the piston, seals, and return spring.
Unfortunately, all of these parts are rusty. That means the bore of the master cylinder is likely pitted. Unfortunately, particularly with old bikes like this one, folks park them and don’t change the brake fluid for years. But, brake fluid is hydroscopic meaning it absorbs water from the air all the time. Eventually the water settles out and corrodes the steel master cylinder and internal parts. 🙁
I use an LED flash light to look inside the master cylinder and it’s not pretty even after I cleaned it out with brake cleaner. So, I will have to replace it rather than rebuild it.
How To Install Front Master Cylinder Piston Assembly
Since I managed to damage the left throttle control housing and bent it I dropped it and the managed to step on it, :-(, I’m going to buy the left side complete throttle control assembly. Consequently, I won’t be able to show how I install the Magura front master cylinder rebuild kit on this project, but I can describe how this is done.
The front master cylinder piston assembly comes in one piece with a front seal and large flat washer that are installed inside the throttle assembly where the front brake lever mounts. The seal fits into a hole in the throttle housing and the master cylinder piston nose fits inside the seal. The flat washer fits in front of the seal to protect it as shown in the beginning of the “Disassemble Front Master Cylinder & Inspect” section above.If I was to install it, I would soak the piston assembly in some DOT 4 brake fluid for several minutes. I would liberally use the brake grease on the seals and put some inside the bore of the master cylinder to make it easier for the piston assembly to slide into the bore.
Then I would put the assembly inside the bore of the master cylinder starting from the return spring end of the piston assembly.
I would secure the piston assembly inside the master cylinder with a new C-clip.
Finally, I would install the front seal into the throttle assembly, attach the master cylinder to the throttle assembly, add the flat washer and then install the brake lever.
Disassemble Rear Master Cylinder & Inspect
The rear master cylinder is attached to a bracket that mounts on the inside of the right rear passenger foot peg bracket. The rear brake foot pedal connects to an actuating lever that pushes on the end of the master cylinder piston. The rear brake fluid reservoir and hose attach to a nipple on the body of the master cylinder. I removed the master cylinder from its mounting bracket and the rear brake pedal when I removed it from the bike.
The rear master cylinder is made by Brembo and mine was manufactured in February 1981.
Rear Master Cylinder Video
I made a short video on disassembling the rear master cylinder and rebuilding it.
Remove Brake Fluid Reservoir Hose
The brake hose is secured to a nipple on the master cylinder with a use once crimped connector. I use my Dremel tool with a cut off wheel to cut through the crimp on the steel band.
I don’t want to cut through the band where it wraps around the hose as I risk damaging the nipple inside the hose should I get too aggressive with the cut off wheel. So I cut the crimped end off the band.
I use a screw drive to spread the ends of the clamp and pull the hose off the nipple.
There is a part number and the bore size of the master cylinder stamped into the master cylinder next to the nipple.
Remove Rear Brake Pedal Actuating Lever Assembly
Next, I remove the brake pedal actuating lever assembly. It mounts on a pin that goes through two bosses on the master cylinder and is secured on each end with an E-clip that fits into a groove of the pin. The pin also secures the rubber boot that keeps dirt out of the end of the master cylinder.
I use two screw drivers to push the E-clip off the ends of the pin and remove them and the flat washer underneath them.
I mount the master cylinder in my vice using the soft jaws. I use a drift to drive the pin out and remove the brake pedal actuating mechanism from the master cylinder.\
Remove Piston Assembly
Unlike the design of the front master cylinder, the rear master cylinder piston is not retained in the master cylinder bore with a C-clip. The actuating lever assembly keeps the piston captive in the bore. I have to drive the piston out of the bore from the threaded hole the brake line screws into.
One suggested method is to use compressed air to push it out. You have to put your finger over the brake fluid nipple while you do this. I mount the rear master cylinder in the soft jaws of my vice and put some shop rags underneath where the piston will come out so it doesn’t smack into the vice or the floor and apply air pressure. In my case, this did not work.
The other method is to use a small drill bit, 3/32 inch works, and push it through the hole at the back of the threaded hole and tap it out with a hammer. This worked for me.
In the above picture, the dust seal parts on the left end of the piston rod are not shown in the proper order. I dropped the piston assembly when I removed it and didn’t put the seal parts on the rod in the correct order. I show the correct order later in the assembly pictures.
Inspection – PASS 🙂
The piston assembly is not rusted so that’s a good sign. All the piston assembly parts are replaced in the rebuild kit. I clean out the bore of the rear master cylinder with brake cleaner and inspect it. There are no signs of rust or pits so I can reuse it.
Polish Rear Master Cylinder Bore
I like to use 600 wet/dry paper to polish the master cylinder bore before I install the rebuild kit. A 1/2 inch drill bit fits inside. I wrap the 600 wet/dry paper around it one layer thick and secure with masking tape. I make sure the end of the bit is about 1/2 inch from the end of the sand paper and fold the paper over the bit to keep the end from scratching the bottom of the master cylinder bore. I run the paper in and out at a slow to moderate RPM while pushing it in and out of the bore. Then I clean it out with brake cleaner and use a strong flash light to inspect the bore. It shines like a mirror which is what I like to see. 🙂
Assemble Rear Master Cylinder Rebuild Kit
Before installing the master cylinder rebuild kit, I painted the master cylinder with brake caliper paint which is resistant to brake fluid. I protected the inside of the master cylinder and the nipple from the paint.
The kit includes a new piston with two large o-rings installed, a packet of grease for the rubber seals, and the front seal assembly that includes a rubber seal, one-time crush ring and flat washer.
The piston has a larger diameter nose on one end with a slight step next to the face of the piston.
The front seal has a side with lettering including “Brembo” (hereinafter referred to as the “Brembo” face) and a part number. It also has a chamfer around the hole. The other side has no lettering and has two stepped flat faces (hereinafter called the “Flat” face).
One side of the one-time crush ring has a flared edge that is rounded. The other side has a straight edge
The flared end of the one-time crush ring slides over the flat face side of the seal so the flare butts against the lip of the seal.
The flat washer goes on the larger diameter nose of the piston and fits against the face of the piston.
Then the seal with one-time crush ring goes on with the “Brembo” face of the seal up against the flat washer and the straight edge of the one-time crush ring on the flat face side of the seal.
The front seal assembly is in a sandwich with the flat washer on one side of the lip of the seal and the one-time crush ring on the other side of lip.
The return spring fits onto the smaller diameter nose of the piston.
Here is the piston assembly with return spring ready to install inside the master cylinder bore.
Install Piston and Assemble Rear Master Cylinder
Here are the parts needed to install the piston into the master cylinder.
Before I put the piston assembly inside the master cylinder, I soak the piston assembly in brake fluid for several minutes.
Then I liberally apply the brake grease onto the two factory installed seals of the piston and on the front seal. I put more grease inside the master cylinder bore with my pinkie finger to help the piston assembly to slide inside the master cylinder.
I insert the spring then the small diameter nose end of the piston into the bore. I put the flat washer on the larger diameter nose, then the front seal with the stiffener ring so the Brembo side goes against the washer. Then I push the piston all the way into the master cylinder to seat it in the bore.
Before I install the actuating lever, I put a small dab of brake grease inside the bronze busing of the lever and on the faces to lubricate it. Then I lubricate the rubber boot with some Ruglyde to make it easier to stretch it over the lever arm and body of the master cylinder
I put the pin through the bushing while pressing the actuating lever onto the nose of the piston. It is a bit tricky to get the hole in the lever to line up with the holes in the bushing.
Then I install the rubber boot over the end of the master cylinder and the plunger of the piston assembly. I found hooking the opening over the lever and using needle nose pliers to pull the boot over the end of the master cylinder works well.
Then I install a new flat washer and the original E-clip on each end of the pin. I use needle nose pliers to seat the E-clip into the groove.
I test the actuating lever to ensure it’s operating smoothly. Then I install the new hose onto the nipple of the master cylinder securing it with a new hose clamp. I slide the other over the nipple in the bottom of the rear brake fluid reservoir.
Replace Rear Master Cylinder Foot Brake Pedal Linkage
The parts are very rusty so I replaced them with new parts.
The 6 mm nut threads onto the threaded end of the turnbuckle. The plate that secures the rear brake pedal return spring goes on next to the nut.
But before I install the plate, I insert the “D” end of the spring into the hole. Then I put the plate on the threaded half of the turnbuckle and screw on the other half of the turnbuckle until it’s snug against the plate.
When I install the master cylinder on the master cylinder bracket and attach the rear brake to the master cylinder, I will adjust the length of the turnbuckle so the rear brake pedal starts actuating the rear disk brake when I push my foot down on the brake pedal.
2020-06-13 Clarified parts order and terminology of the dust seal and one-time crush ring.
Brook, thanks for more great content. What kind of clamps did you use for the new hose?
I used the BMW part, part# 16 12 1 116 458 and the pliers needed to crimp it. I got both at Euro MotoElectics (www.euromotoelectrics.com).
I hope that helps.
On the rear master cylinder, what keeps the washer, seal and one time crush ring in place? There was no mention of seating the washer, seal and crush ring in your instructions or the video.
The lever secured by the pin with two snap rings on the end of the master cylinder holds all the parts in place inside the bore.
Following the video, I installed the components. Once I put on the lever, I shook the master cylinder and the seal and crush washer fell back onto the lever. I have looked closely at the crush washer to make sure the flat side and slope of the crush washer was correct.
I really enjoy the videos. Thanks Brook
So it sounds like “all is well”. That’s good.
No, I have to be missing something.
I have made sure the Crush Washer is facing the proper direction, and slid the piston back into the bore.
The washer, seal and crush washer slide into the bore.
When I put the arm back on, the crush washer and seal slide out to the edge of the activating arm. I have a photo but do not see a location to upload it.
Hi love all the videos, very informative. I am going to install piston kit in my rear master cylinder but I am not sure how the seal and washer stay in place, the front master cylinder uses a circlip so I am wondering how the rear cylinder seals are held in place? Also you call the metal ring a ‘crush’ washer so that makes me think you have to do another step (crushing it?) to hold it in place? If only you lived in my street and I could come and ask you!
Thank you for the kind words.
You don’t crush the washer that fits around the rubber seal. IIRC I pushed the piston with the seal and washer into the bore of the cylinder and then installed the lever arm to hold it captive. The piston and seal stayed put in the bore of the cylinder.
I hope that helps.
I was questioning the sane thing a few weeks ago. I ended up using a 9mm deep well socket to get the crush washer to seat. I put the master cylinder into the vice, clamping it where the bolts attach to the bracket. Then I pushed the piston, washer and crush washer in with the deep well socket. It took some effort. After installing I bench bleed the cylinder but that did not hold. I mounted the cylinder and attached a 40 mm brake line and vent it around so it drained into the reservoir and bled it again I disconnected that line and held my finger over the port and quickly attached the brake line. From there I was able to use a MityVac to evacuate the air and partially bleed the system. I completed the bleeding the line via the typically bleeding process of pumping the brake and bleeding the air from the caliper.
Hope you are still monitoring this video chat.
I have cleaned up the rear master cylinder bore and with decent seals (only 58k miles on this bike), I move to reassembling & find with the cylinder bottomed out, the actuating arm pivot pin hole does not line up. It remains abt .5mm short but even if I could slide the pin in place, with two points of contact tight, (the contact point on the cylinder end & the pivot pin), there will be no movement for the actuator to actuate.
I am in a conundrum. I have reassembled this cylinder previously and tho it operated correctly, I did not have a seal. So I have redone the process and now find myself thwarted. I have nothing extra/accidentally dropped into the bore, but the cylinder suddenly appears to be too long. Argh!
Hmmm. You did note that there are two different versions of the rear master cylinder each with it’s own set of rebuild parts? One is for the master cylinder that has “15” stamped on it:
34 31 1 237 233 SET: REPAIR KIT BRAKE MASTER CYLINDER – D=15,87 (although not stated, I assume its up to 09/80 based on the 2nd part description)
And the second is for the master cylinder with “14” stamped on it:
34 21 1 242 791 SET: REPAIR KIT BRAKE MASTER CYLINDER – D=14,29 (from 09/80) 0.07
Other than than, I currently don’t have a suggestion as to the cause of your problem.
Thanks for the info again. I followed the procedure on 2 rear master cilinders, but i can’t bleed either of them: the brake fluid in the reservoir does not go into the MC although the hole has been cleaned and tested. No idea what I’m doing wrong. The hole with the plastic has been removed to check but no issue there.
Any idea would be extremely welcome.
Thanks in advance and best regards,
I would remove the rear steel brake line. Then operate the lever on the master cylinder to see if it is pumping. If not, the problem is in the master cylinder, the line from the fluid reservoir or in the reservoir. To check the line and reservoir, you have to remove the crimped clamp on the brake line at the master cylinder to diagnose if the problem is in the line or the reservoir.
If the reservoir and lines are working, then it could be the seals on the master cylinder maybe be leaking or the bore is not able to seal against the seals. There are two different size master cylinders, so you want to be sure the rebuild kit is for the correct size bore on your master cylinder.
It it pumps fluid, attach the steel brake line and detach it from the rear caliper. Pump the master cylinder lever and see if fluid comes out. If so, the line and rubber hose are not blocked. If it doesn’t come out, there is a blockage.
Then I’d check the rear caliper. With the brake line removed, I’d press the pistons with my fingers and see if they move a little bit. If not, something is blocking them inside the caliper.
In short, start at the fluid reservoir and prove/disprove each component works until you find what isn’t working.
I hope that helps.
Hi Brook, it’s definitely the master cylinder. I’ve detached it and I see it pumps out, but doesn’t refill from the reservoir (which has no blockings) when retracting. Even air pressure into the opening where the line of the reservoir is attached doesn’t get through. Very strange I would say, or is this normal behaviour? Air goes through if the cylinder is not inserted.
I would remove the piston from the master cylinder. A seal may be blocking the small holes in the bore that let in fluid from the reservoir. Perhaps the seal lip ended up out of place when you inserted the piston. With the piston out, you can also verify if the hole that lets fluid into the bore is blocked.
Hi Brook, thanks for your reply,. My third and last MC to be used in my following project worked. So I’m going to use this one for my present project and later i will try to check the faulty ones with the info you gave me. It must be the seals….
Have a great weekend
Hi Brook, the piston doesn’t retract far enough. This is provable because i re-use the one time use ring which blocks the piston and pushed it in to far so it would hold. Working out another solution to avoid buying a repair set just for the one time use ring. The other seals are still perfect. Thanks for your valued comments.
Hi Brook, it’s solved: combination of piston not retracting far enough and a blocked hole. Not the big hole, but the extremely tiny one in front of it. It’s even very difficult to see looking inside the cylinder. And very difficult to clean. But i managed. Thanks again and keep up the good work!
Best regards, luc
I’m on the same boat w you, Luc. R100RT & the rear has been difficult to bleed. I had Apple Hydraulics, (NY), rebuild the MC & will soon put a seal kit in the caliper, but so far it’s been a challenge.
And any idea on why it didn’t work?
I’m going to test my 3rd MC today…