1977 R100RS

[jsey]

Hi all,
I just got finished installing a new rear brake pipe and master cylinder. I must say that whole affair, along with the pedal linkage is completely inaccessible. Ridiculous engineering in my opinion. Before you get everything just right the whole assembly gets installed and removed several times which of course means that you have to re-bleed. The original MC on my bike was 15mm. My threads were messed up so I needed a new MC. The only thing that is available now is a 14 mm MC. I was told that it had more power but a longer throw. There’s to much travel on the pedal to such an extent that you find yourself stretching to come to a strong rear wheel brake. Anybody else here experience this problem?

 

[robsryder]

I don't understand. I have a 77 R100RS and my rear brake is a drum unit actuated by a rod attached to the rear brake pedal. The front brakes are twin discs and are operated by a combination of cable (from handlebar to under-tank master cylinder) and hydraulic brake lines (from master cylinder to calipers on discs). At the time BMW was using calipers from ATE; it was later that Brembo became the preferred supplier of calibers.

Some folks have converted their bikes to handlebar master cylinders. Perhaps someone has modified your bike to have a rear disc brake. In which case, who knows what you have.

OEM parts can be found via on-line sources, such as MaxBMW's parts fiche -

MAX BMW Motorcycles - BMW Parts & Technical Diagrams - R100RS 77-84 (2474)

BMW Parts for your R100RS 77-84 (2474) Production: 05/76-10/84 USA

shop.maxbmw.com

Oak and Snowbum have previously discussed airhead brakes, including (as I recall) use of the properly sized piston on a master cylinder for single and dual brakes. Alas, Oak's info is largely inaccessible because he did not like having it on-line. Snowbum's info is available on-line, but may require several readings while fully sober -

BMW Airhead BRAKES and other motorcycle brakes; modifications; overhauling ATE under-tank master cylinders; also sidecar brakes

BMW motorcycle brakes information, bleeding, flushing, service, snowbum, sidecars,snobum

bmwmotorcycletech.info

Anton has somewhat more readable info on BMW bikes; his info on airhead brakes is here -

BMW Airhead Master Cylinders

Master cylinders and controls for BMW Airheads.

largiader.com

There was also a past discussion about master cylinders for airheads on ADV. [Note: not all info on the internet is perfectly correct ]

BMW R100/7 Front Brake Advice

I know there are many other threads on this, and I've read a lot of them, but since many are old/outdated (kit links broken/discontinued, pics...

www.advrider.com

Yet another resource is the archived info on the IBMWR website. These tend to be owner generated procedures and information.

Airhead Tech Articles - The Internet BMW Riders

BMW Type 247 motorcycle maintenance, mostly for 1970-1995 bikes, but some older models also. R50, R60, R75, R90/6, R90S, R65LS, R100, R100S, R100RS, R100RT

ibmwr.org

 

[stevenrankin]

There was a discussion about a 77RS with disc brakes on the Airheads forum a month or so ago. BMW very well could have started building 78RS bikes in the later part of 77 or used some "new" features on late year 77RS bikes.

I had a 78RS with the rear disc, and hated it for the same reason posted here. I did get my rear brake working properly and it worked but I prefer the drum brake.

On my RS there was a turnbuckle or adjustable link between the pedal and the master cylinder. Try adjusting that to take up some of the gap in the pedal. As for bleeding, you really have to bleed the caliper off the bike. Put a block of wood between the pads and turn it upside down or at least so the bleeder is at the highest point, then get someone to help out by pumping and holding the pedal. Good luck, St.

 

[bremoit]

Installed one of the new 14mm MC on a 78 RS recently.

Once the ballet of bleeding the line was complete, no issues and works just fine.

You might consider using a syringe to "reverse bleed" and push fluid thru the caliper to the res, as part of the process---making sure the caliper bleeder is at the highest point, as others have noted. The reverse bleed and standard vacuum pump method together made short work of this for me last fluid change.

Others have modified the pivot point in the brake lever to compensate for the 14mm MC, I found that was not needed.

Also, helps to lubricate/spray/cover the entire mechanical linkage assembly with tri-flow or a black graphite dry lube so all the parts are moving freely w/ no binding.

This set up is a bitch to service with cacophony of parts.
Once set up it does work well.

Good luck---stay w/ it and you'll get it working right.