1980 R100RT Brake rebuild questions

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t3tommy
Posts: 3
Joined: Tue May 24, 2022 3:42 pm

1980 R100RT Brake rebuild questions

Post by t3tommy »

Hello all...newbie here with my first BMW. Got a 1980 R100RT with about 60,000 mi with a buddy we're trying to get the bike running again after sitting since 2006. Biggest problem is the brakes...fluid gone from both reservoirs and master cylinders. Lines clear and OK, caliper pistons not stuck, pads OK, rotors OK (lightly scored but should at least work), but need to rebuild both front and rear master cylinders. Front came apart with a little persuasion and will be honed lightly to try to save the bore (no major corrosion), but seals and orings hard and useless . I've searched the forum for other threads about early BMW disc systems, and will be buying rebuild kits for both front and rear mcyls. Problem is that we can't get the rear mcyl apart...can't get the piston/seals/spring out. Tried several times, but "no joy". Piston moves, spring has pressure, but we can't figure out if there is a circlip or something that holds the innards inside. Clymer BMW book says virtually nothing about dismantleing/rebuilding the rear mcyl. I think it's a Brembo unit...the rear caliper is a Brembo.
So...anyone done this job before and can help us out? Also...where is the best place to buy brake mcyl rebuild kits? Actually, since we're so new to this 42 yr old bike and will need more parts eventually, can anyone point us to reputable vendors for parts? I've been looking on Ebay, but .....never know if you can trust people there.
Thanks in advance for any help/ideas/directions from forum members. Tom and Russ

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srankin
Posts: 481
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

Re: 1980 R100RT Brake rebuild questions

Post by srankin »

I can't give you any advice or help with your rear master cylinder. I can give you a list of sources I have found over the years to be top notch. Perhaps looking the master cylinder up on Max's microfiche it may show something like a clip or snap ring in the set up.

Max BMW, their online microfiches are top notch and up to date on parts available. Also, they carry or can get a ton of parts for most airheads: https://www.maxbmw.com
EME, Euro Motoelectrics: https://www.euromotoelectrics.com/.
Boxer Two Valve: https://www.boxer2valve.com
Siebenrock in Germany: https://www.siebenrock.com/
Motobins in the UK: https://www.motobins.co.uk
Motoworks in the UK: https://www.motorworks.co.uk
Motorrad Electrik: http://www.motoelekt.com

I am not going to comment on official BMW parts versus aftermarket. Good luck and welcome to the airhead world. St.
Owner of a 84, R80RT and 78, R100RS

bremoit
Posts: 14
Joined: Wed Jan 20, 2021 2:11 pm

Re: 1980 R100RT Brake rebuild questions

Post by bremoit »

Brook Reams has a good video and blog post about this. Simple search will turn it up.

It is a Brembo brand part.

Basically use a drill bit or thin barrel screwdriver to push out the piston from the open end (threaded end) of the m/c. There is no circlip at the piston end of the m/c. The seal on the piston end is pressure fit (one time use crush washer) and holds the piston in place.

There is a proper procedure and orientation when assembling the new seal on the piston end of the m/c, which Brooks covers in excellent detail as well. Note the seal is a "one time" deal, so get it right the first time. It can be difficult to get the seal/washer to stay in place if you F it up the first time. It helps to find a socket and use it as a driver when putting the new seal assembly in .

EME almost always has the best price and fastest shipping.

You'll need to check if you have the 15mm (early) or 14mm (later) m/c. 1980 was a transition year for that part, at least as the MAX parts fiche notes.

The m/c should be stamped with one of the numbers AND when you get the piston out you can tell which one it is by comparing pics of the piston you have with the rebuild kits on EME...the later 14 has a smaller/thinner end where the spring fits (if I recall) than the earlier 15mm.

The bleeding process can be tricky and getting a good lever feel, can take some time. You might try reverse bleeding with a syringe and/ or applying some over night pressure to the pedal (wedge between the brake pedal and foot rest) to force out any remaining air after you bleed the system. That last step can make a difference, especially with reducing the pedal travel and taking out some of the "soft feel".

Cheers!

t3tommy
Posts: 3
Joined: Tue May 24, 2022 3:42 pm

Re: 1980 R100RT Brake rebuild questions

Post by t3tommy »

Thanks to both repliers with excellent information. Bremoit...you are the guy I'd hoped I would find here. So it's a crush washer that holds the piston in there...that explains it. I'll check out the Brook Reams video, too. Also, thanks for the very valuable info. on piston size in the mcyl., as I'd run into that during parts searches and wondering why there were 2 sizes of mcyl pistons. The info about 1980 being a transition year also explains why this bike has some "old" stuff and some"new". I already knew that it was the last year of "points type" ignition, and I thought it was strange that front brakes were ATE (?) and rears were Brembo. Also learned that 1980 was the year of the failing "snowflake " style wheels. This is going to be an interesting brake job .... and I'll probably order thru EME even though they're a little more $ than others. I'll keep you all updated on how the process goes. Thanks again from Tom and Russ.
Cheers! indeed.....

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srankin
Posts: 481
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

Re: 1980 R100RT Brake rebuild questions

Post by srankin »

I have never figured out why BMW installed a rear disc brake Brembo unit on the years of bikes that had front ATE brakes?

Yes, a cross over year for BMW can be confusing, lol try sorting out a wiring question some times.

Brook's videos can be found at:

https://brook.reams.me

St.
Owner of a 84, R80RT and 78, R100RS

t3tommy
Posts: 3
Joined: Tue May 24, 2022 3:42 pm

Re: 1980 R100RT Brake rebuild questions

Post by t3tommy »

Brook Reams Airhead Garage is THE PLACE for great info. on airheads! Me and Russ went ahead and pounded out{!) the pistonon the rear mcyl with a 3/32 drill bit as a drift. I still can't believe that there isn't a circlip holding the piston inside... seems like a real screwy setup. How do we know how far the new one goes into the mcyl bore so that it is situated so the seals pump and release fluid? From questions other owners have asked Brook, it seems that many of us have had problems/questions about this setup. Looking at the manufacturing code stamp on our mcyl, it was made in July 1978! And it isn't stamped with the bore diameter like the one Brook shows . Anyhow , we got the darn thing apart and the bore measures 15mm (the earlier part). Why oh why did the part sit on a shelf for 2 years before BMW installed it on a new bike? I've always heard about "German Engineering" and how BMW's were so excellent, but this whole rear disc brake setup is just plain goofy. The rear disc setup on my 1978 Suzuki GS1000 was easy to service and worked great. Too bad BMW used Italian brake parts.....they look like they're made for a Fiat!(Not that that's bad....but...)
Anyhow, ordering parts from EME and on to the next step in the process....will keep you updated. Thanks again for replies!
Russ and Tom

bremoit
Posts: 14
Joined: Wed Jan 20, 2021 2:11 pm

Re: 1980 R100RT Brake rebuild questions

Post by bremoit »

The rear disc on these does leave a bit to be desired in a number of areas and it was a rather short lived.

The drum rear made a triumphant return in the mid 80's and lasted to the end of the run.

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