And now the Brakes

[raymr]

I've been working diligently on the R65, getting it ready for state inspection. It's running great, mostly.

Today the brake master cylinder was making squishing sounds and looking underneath found a slow drip at the end. When I looked online, they sell 13mm and 14mm replacements. Mine is stamped 15mm.

Do I have the wrong size to begin with? If so, what's the best size for good brake feel? This one did feel a little hard to work, but not unreasonable. Can I get away with just replacing the piston?

[bruce_launceston]

The 15mm is standard and ideal for dual front discs.
If you have a single disc the 15mm master cylinder is the wrong size and wouldn't give much feel.

[Rob Valdez 79 R65]

Did all of the mono-shock bikes come with dual-disk front brakes?

My '79 single-disk has a 13mm master cylinder bore, but I think the two brake systems are apples & oranges.

[Barry]

but I think the two brake systems are apples & oranges.

I agree. Because I think Ray's bike will have larger diameter caliper pistons which makes comparison of master cylinder diameters with the early bikes difficult. To make any sensible comparison you need to calculate the ratio of master cylinder to caliper piston area and even then the handlebar lever ratio may be different. Is the master cylinder/lever assembly and therefore spare parts the same for the later bikes ? I don't know.

[raymr]

It's a single 48mm Brembo caliper. From what I'm reading, 13mm seems to be the right m/c size. Now I'm wondering if the bike used to have a dual disk setup and the 15mm m/c is left over? It would be strange if it came from the factory with the current configuration.

[Barry]

From what I'm reading, 13mm seems to be the right m/c size...... Now I'm wondering if the bike used to have a dual disk setup and the 15mm m/c is left over?

If what you are reading about single disk models being 13mm as stock definitely refers to post 85 models  then it would make sense that a dual disk conversion might use 15mm.  On the other hand ignore anything you read about R65 master cylinder sizes that doesn't clearly state it refers to post 85 models.

Going back to your original question. You may get away with just a new piston assembly if there is no severe corrosion in the bore. If any imperfections are minor you could try lapping the master cylinder bore with very fine grit wet and dry paper on a wooden mandrel. Many people have done it with success as the seals have sufficient flexibility that the bore dimension is not critical to the nearest half thou.

[Bob_Roller]

I've used this method on two master cylinders and it worked quite well .

Get a hardwood dowel from a home improvement store, use a hacksaw blade to cut a slot in one end, cut a piece of scotch brite type material, put it in the slit, chuck it into a drill motor and use a slow speed until you see how it's working .

If your master cylinder won't clean up and is deeply pitted, there is a repair shop called White Post Restorations that wil put a brass sleeve in it .

 http://www.whitepost.com/brake.html

Don't know if it's cost effective or not, I checked with them in 1991 for my '81 R65 and the cost was $125 then .

[raymr]

So it looks like I have a choice between repairing the old unit for 60 bucks and keeping the hard brake feel, or springing for a new correct size one for $200 that might offer better modulation, i.e. safety. The kids' college is nearly paid off so I'm feeling I might splurge a little..

[raymr]

So the new master cylinder came with a brass washer and a rubber sealing ring around the stem of the piston. None of the manuals I looked in show these pieces.

I wasn't sure what they did but I installed them thinking maybe they were added later to address a dust/water infiltration problem. Well the rubber seal didn't quite bind the brakes but made the lever action sluggish. I took the pieces out and the front brake is now acting like it should.

Why are they included with the m/c? Hopefully not just to confuse people??

[Red_Hen]

Ray
The thin brass washer goes between brake line & where it screws into m/c to seal connection. Not sure about rubber piece w/out seeing pictures. Spiegler stainless brake lines &.proper bleeding helped brakes immensely. I'd overhauled Brembos & installed new pads & Spiegler floating rotors.

Hope this helps.

[Bob_Roller]

The rubber 'boot' is a dust/dirt seal for the master cylinder, I'll look when I get home, but I believe the brass washer goes over the stem of the piston and then the rubber boot goes over it .

[raymr]

Ken,
Those washers I think are copper and I have them on both sides of the brake line connector where the bolt goes through. This mystery washer is bigger and the rubber piece is more of a boot as Bob says. I'm more inclined to think it will gunk up over time, than offer any real protection since the stem end is pretty well enclosed once it's mounted.      

Ray
The thin brass washer goes between brake line & where it screws into m/c to seal connection. Not sure about rubber piece w/out seeing pictures. Spiegler stainless brake lines &.proper bleeding helped brakes immensely. I'd overhauled Brembos & installed new pads & Spiegler floating rotors.

Hope this helps.

[Red_Hen]

did you check in Clymer book or Real OEM site?  Also, Motobins has good pictures too.  

It seems, going by memory, that a washer came with the kit I purchased to overhaul my front m/c.  

I'm sure someone here will come up with the answer.

[Milo_357]

So it looks like I have a choice between repairing the old unit for 60 bucks and keeping the hard brake feel, or springing for a new correct size one for $200 that might offer better modulation, i.e. safety. The kids' college is nearly paid off so I'm feeling I might splurge a little..

$200?  I'm not sure I paid that for a new one from Bob's BMW.  Give them a shout.  I bought 15mm earlier this year.

Dell

[raymr]

They are all $198 give or take a few pennies.

BTW I figured out that the old rubber seal was still stuck in the brake handle housing. It was just hard to see, and that explains why the new one didn't exactly fit in there right.

All is well now after I pried out the old one and replaced it. Live and learn.

Plus, the brake feels wayyy better with the smaller m/c.