Optimum master cylinder size for twin disks?

[zebedee]

I've got a 1984 late twin shock R65 with factory single disk which I'v had since 1987/4000km.

Over the years I've picked up a second disk and now a right side caliper. The current 12mm master cylinder has internal corrosion (despite having been stainless steel sleeved in about 1995), so now is the time to change to twin disks.

I believe the factory master cylinder for the R65LS or R65 with optional second disk was 15mm, but other sizes are also available. Has anyone tried a 14mm master cylinder?

The factory plumbing seems to have involved a T up near the headlight and two brake lines. Is there any reason I can't put a short line from the bleed port on the left caliper into the right caliper, and bleed at the right caliper?

Graham in Oz

[Bob_Roller]

You can route the line for the right caliper from the left caliper .

You just need a longer 'banjo' or hollow bolt and an extra crush washer .

My bike came with twin front discs from the factory and had a 15mm master cylinder .

The 14mm master, will  have a longer travel to provide the same fluid displacement as a larger bore cylinder, don't know if that will cause any problems in real life use or not .

[Julio A.]

How could you tell the displacement of the master cylinder?
I really don't know what size is on mine.

[Lucky_Lou]

Julio the size should be stamped into the casting under the reservoir
My 79 twin disc is 16mm Brembo master cylinder with twin ATE calipers.
There is a brass splitter block under the headlight you can see it in this pre rebuild photo .
Pretty good braking really
Motorworks part number
BRA 02355
Lou

[zebedee]

How could you tell the displacement of the master cylinder?
I really don't know what size is on mine.

The bore in mm is cast into the underside of the master cylinder. As far as I can tell the options are 12, 14 and 15, though Lucky_Lou says he's got a 16. I've got a bare 15 somewhere here, but no innards. I might see if it can be sleeved down to 14, otherwise put a new 15mm piston and seal kit in it.

[zebedee]

You can route the line for the right caliper from the left caliper .

You just need a longer 'banjo' or hollow bolt and an extra crush washer .

My bike came with twin front discs from the factory and had a 15mm master cylinder .

The 14mm master, will  have a longer travel to provide the same fluid displacement as a larger bore cylinder, don't know if that will cause any problems in real life use or not .

There is no banjo fitting at the caliper on my bike, the brake hose fits straight into a threaded hole in the caliper.

The hose and the bleeder are both the same thread.

Before I replaced the rubber hose with a steel braided one, I had a little trouble with excessive lever travel with a 12 and one disk. As I said above, I've realised I have a bare 15mm casting, so I guess thats getting built up.

[bruce_launceston]

Stick with a 15 mm bore, the standard R65LS twin disk setup has good feel.
The standard brass splitter fitting behind your headlight should have plenty of blanked off outlets where you can run a separate left and right hose from.
I have twin disks and there is still a blank spare, I guess it is for a older style front brake light switch.
If you buy a new hose set, mine is a HEL set, you can get rid of a lot of clutter from the lower fork leg as the hoses go straight to the brake calipers. This set utilises the existing splitter behind the headlight.

[Rob Valdez 79 R65]

Here is the splitter:



And here it is, mounted:

[nhmaf]

I was going to suggest trying the 14mm if you like a bit more travel and "feel" in your front brake.  My LS has the 15mm m/c with twin Brembos and it goes from nothing to full lock in very short distance.

I'm used to it, but not everyone cares for such rapid/abrupt braking.

[Julio A.]

Mind I ask, I checked the Master cylinder on mine and at has the number 16 stamped under it. But I only have a single disk.
Does this mean My R65 has originally two disks on it before?

[Barry]

Mind I ask, I checked the Master cylinder on mine and at has the number 16 stamped under it. But I only have a single disk.
Does this mean My R65 has originally two disks on it before?  

It either means your bike had  two discs or the master cylinder has been replaced at some point with one from a twin disc bike.

You must need strong hands to make that brake work I wouldn't have thought it feels very safe. For a single disc the stock master cylinder should be 13mm for the 78 - 80 bikes and 12mm for the 81 - 85.  I'd go for the 12mm unless you are thinking about fitting twin discs.

[DgM]

I twin disc'ed my 1983 R65.  For complete peace of mind have both calipers rebuilt, source caliper spacers - there are at least three sizes, use correct, return rest - put on NEW 15mm Master Cylinder, source factory splitter, install NEW hoses  - SS cool, ATE factory fine, enjoy another 20 years+ with no braking issues save for pad replacement.  Twin discs provide enough stopping power to pull a stoppie, though as a BMW R65 rider you would never resort to such nonsense.  Of course...