R65? Master Cylinder part identification help needed

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.

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?

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).

kheerema said:

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.

Click to expand...

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.

R65 Master Cylinder

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

found it!

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

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