R65 front master cylinder bore?

[darrylri]

All I know is that with a 15mm bore on my dual disk setup, the brake lever went to about a 1/2" off the handlebar before there was any brake action.  They worked fine otherwise, but I couldn't do two-finger braking, as I do on my other hydraulic brake bikes.  

When I bought and installed the correct 16mm MC, the braking point was well off the bar, just like on my other bikes.  

And while the ATE brakes do stop the bike in a reasonable manner, they just don't have the feedback qualities of the later Brembos.  At least, IMHO.

[zebedee]

Julio,

That sounds like you're saying you have the rectangular reservoir?

I don't know, but I'd imagine that BMW made single disc versions with the rectangular reservoir, but they would have had a 13mm cylinder bore.

The single disk version of the master cylinder with the rectangular reservoir as fitted to 1981 through 1984 R65s is 12mm.

[zebedee]

All I know is that with a 15mm bore on my dual disk setup, the brake lever went to about a 1/2" off the handlebar before there was any brake action.  They worked fine otherwise, but I couldn't do two-finger braking, as I do on my other hydraulic brake bikes.  

When I bought and installed the correct 16mm MC, the braking point was well off the bar, just like on my other bikes.  

And while the ATE brakes do stop the bike in a reasonable manner, they just don't have the feedback qualities of the later Brembos.  At least, IMHO.

This sounds like your rubber brake hoses have got old and soft and are expanding under pressure, using up some of your lever travel. Going to a larger master cylinder allows you to move more fluid but at a lower pressure, compensating for the expansion of the soft hoses but reducing the braking force at the calipers for a given lever force.

[darrylri]

All I know is that with a 15mm bore on my dual disk setup, the brake lever went to about a 1/2" off the handlebar before there was any brake action.  They worked fine otherwise, but I couldn't do two-finger braking, as I do on my other hydraulic brake bikes.  

When I bought and installed the correct 16mm MC, the braking point was well off the bar, just like on my other bikes.  

And while the ATE brakes do stop the bike in a reasonable manner, they just don't have the feedback qualities of the later Brembos.  At least, IMHO.

This sounds like your rubber brake hoses have got old and soft and are expanding under pressure, using up some of your lever travel. Going to a larger master cylinder allows you to move more fluid but at a lower pressure, compensating for the expansion of the soft hoses but reducing the braking force at the calipers for a given lever force.

Nope.  PO put stainless braided lines on it.  And with the correct 16mm MC on the bike, the brake lever works correctly.

[Mike V]

Darryl,

I replaced my original 16mm master cylinder with a 15mm master cylinder on my 81 R65.  I have the dual ATE discs up front and replaced the original brake lines with stainless through Ted Porter's Beemershop recommendation and supply.  I researched this the best I could and am more than happy with the resulting function of my front brakes.  The larger master cylinder gives a bit more of a wooden feeling at the lever but still has good adequate feel and feedback (modulation).  With adequate bleeding the lever travel is 1/2 throw to the handlebar.  I feel like I could stand the bike up on it's front wheel if I had that kind of skill.  I'm attaching the link to Mike Morse' brake ratio chart for you to view.  Take a look - it's very helpful and will give you a good direction of different combinations.

http://www.vintagebrake.com/mastercylinder.htm

-Mike V.

[darrylri]

Well, I don't know what to say about it that I haven't already said.  

With the 15mm MC, even with a careful brake flush, the brake lever comes back so far to the bar that it would trap my two fingers on the bar before ANY braking would occur.  The braking action was otherwise acceptable.  As the MC has no adjustment for the lever position makes this unacceptable to me.

OTOH, when the correct, original 16mm MC is mounted, the lever moves back a bit and then the braking action begins, and again, the action seems acceptable.  

I can't really tell the difference in how much "feel" there is; both MCs seem to be somewhat wooden relative to what I am used to on more modern bikes (R1100RS, R1200ST).  The braking delivered by either MC is much closer to what I am used to getting from my R90S's dual ATE (single piston) brakes (which also have braided steel lines).  

[Bob_Roller]

My '81 R65 came from the factory with the 15 mm master cylinder, I'm the original owner, and the only person that has worked on the bike since I took delivery of it 30 years ago .

I have about .5 inch (12mm) lever travel with 1.5 inches (37mm) clearance between the lever and hand grip with the brake fully applied .

[Mike V]

To clarify my post and support Bob's mention, my 81 R65 came with a standard OEM 15mm master cylinder.  I replaced the destroyed original 15mm (bore pitting) with the larger 16mm master cylinder.  This has worked well for me.  Couldn't ask for better brakes on my R65.

[Vegasrandall]

mine is a square reservoir 13 mm and its a single disc 1980

[Mike V]

With the 15mm MC, even with a careful brake flush, the brake lever comes back so far to the bar that it would trap my two fingers on the bar before ANY braking would occur.The braking action was otherwise acceptable.As the MC has no adjustment for the lever position makes this unacceptable to me.

Darryl,

I struggled with the lever position (compressed) on my rebuild also. I felt as though it was too close to the bar.  I continued to bleed the system so many times I probably went through a full liter of brake fluid.  I found the more I bled the system the more lever I obtained.  Each time I saw microscopic bubbles surfacing in the reservoir.  I continued the bleeding process for probably two days since i was unsure all the air was removed from the system since I completely rebuilt both the calipers, new lines and new MC.  I tapped on the components, turned the bars fully to the left, bungied the lever and let sit.  Repeated the bleeding over and over.  I ended up with sufficient lever travel not pinching my fingers during hard braking. My lever travel pretty much equals Bob's posted values.  

[darrylri]

The picture is from realoem.com, note item #2, from 09/80 to 09/81.  

I first bled the brakes myself, but then a good friend, who is a former chief mechanic at a BMW dealership, visited me and he bled the unit as well.  I seem to recall using a zip-tie to hold the lever under pressure.  Anyway, the lever position never moved forward.

I bought the 16mm MC from Motobins, and after installation, it set up immediately to the correct position.

[Bob_Roller]

This is the first I've seen RealOEM, or the BMW parts catalog specify the 16 mm master cylinder for bikes manufactured from 09/80-09/81 .

This is a relatively recent change, in the last year or so .

They used to have both the 15 and 16 mm master cylinders listed in that picture .

If I ever have to replace the part I'll get the 16 mm .

[R65Guy]

Just to add to the discussion... my 1981 came from the factory with 16mm rectangular master cylinder and dual ATE calipers.  According to my serial number it was manufactured in 10/80.  Braking feel is pretty much as Darryl describes it.  I replaced the MC a couple of years ago and replaced the rubber lines with stainless but the 'feel' really didn't change dramatically.  Adequate but not spectacular compared to modern systems.  Back in 1981 they were the best brakes I had ever experienced on a motorcycle - much better than most other bikes of similar vintage.

[Bob_Roller]

I'm wondering if my bike, being built at the model year change over , it was made 09/80, the ninth bike off of the production line (serial number 6385009) for the '81 model year for the North American market has anything to do with it .

[Vegasrandall]

just a shade tree estimate tells me it would cost more to add a rotor than what I paid for the bike.