R65? Master Cylinder part identification help needed

[kheerema]

I updated my R90/6 to run dual ATE calipers and picked up a BMW handlebar mounted master cylinder on ebay with a smaller bore for the dual calipers. It plugged right into the switch controls and the lever is identical to the stock R90/6's.

I am having a brake fluid leak right at the resovoir cap. Removed the cap, found the rubber diaphram/seal under the cap in need of replacement.

Problem is, I'm not exactly sure what year or bike the M/C came from. It has a round fluid resovoir, and it calls for "DOT 4" on the cover.

I called the dealer, and they weren't exactly sure, thought maybe R65? He looked it up and said I'd have to buy the whole master cylinder to get the rubber diaphragm/seal! I checked some parts fiche, but they are showing a rectangular master cylinder for the R65?!

Can anybody help me identify this Master cylinder? I don't have a pic right now, but I'll get one tonight if it will help.

[crazydrummerdude]

I read somewhere that everything BMW has a part number on it. Can a dealer search by part number?

Are you sure it's a BMW part?

[boxermaf]

I don't think that it wasn't just the R65 bikes that had the round m/c on the handlebars, but the R65 does have some unique brake parts. The R65 models had these round m/c units on the 79 and 80 model years, switching to the rectangular m/c in the 1981 model year.
One can easily get rebuild kits and/or diaphragms for the rectangular units from dealers, and online places like capitalcycle.com The round units are not considered to be as rebuildable, and there are fewer parts available for them I have found, unfortunately. I would try calling capital cycle or the parts place of your choice and talk to them about the part that you have, and perhaps they can help you find a part that will fit, but I have a sinking feeling that you may have to buy the whole unit (round m/c units can still be bought new, rectangular ones can also be bought new but also one can buy repair kits/parts for them more readily).

[GlobalRider]

I updated my R90/6 to run dual ATE calipers and picked up a BMW handlebar mounted master cylinder on ebay with a smaller bore for the dual calipers.

I had an R65 with the dual ATE calipers and it takes a master cylinder with a 16 mm piston. The single caliper master cylinder has a 136 mm piston. The piston diameter is cast into the master cylinder housing near where the brake hose connects.

Mine had a round cap on it.

Check the MAX BMW on-line parts catalog.

[lostboy]

For what it's worth, only the R65 (unless you count the R45) had a round master cylinder resevoir. The diaphram is still available. 32721238387

[kheerema]

Thanks guys,

I was able to find the diaphram on the max bmw parts site that Global Rider posted. Now we just have to see if they have it in stock :thumb

THanks everyone for the help!!!!!!!!