T H E O L D G R E E N B E E M E R
RIDING A 1974 BMW R90/6 FLAT TWIN
BRAKE PERFORMANCE, PART TWO
I forgot to mention - a larger main cylinder is necessary for dual brakes. For a twin
caliper setup you want a 17mm version ( R80/7 donor bike ) the single disk R90
brake is 14mm diameter. Some folks do use the 14mm cylinder but travel of
the hand lever becomes a lot longer than. Way too long, in my view. I found
a good used one, luckily bore and piston where in great shape. This is not
always the case as these things corrode when brake oil was not changed
on a regular basis. A good cleanup and a quick bore hone was all that
was needed. I blasted off all the old paint and coated the outside in
boiled linseed oil, a very good old style anti corrosive. New seal
on the piston and filled it up with DOT 5 sillicone brake fluid.
Various sources told me never to use silicone brake fluid in this system!
Pig headed as I am, I did, and use it over a decade by now. I have not
experienced any problems at all. Only benefits, no more corrosion
Outside coated in boiled linseed oil. Inside silicone oil Dot 5
BY THE WAY, SMALL DETAIL
The BMW factory found a cunning way to introduce water into the brake system. Have
a close look at the brake fluid reservoir ... see the small hole in the cap, that's for
letting air in and out. The thing is, inquisitive raindrops that manage to crawl
under the tank can enter that same little hole as well. BMW managed to
forget to fabricate a rubber diaphragm under that cover. Modern
handle bar mounted units do have such a diaphragm. This is
actually a Car type reservoir. Now to prevent water from
getting in I just placed a thick O ring and a second
cap over the original cap and used a piece
of electrical tape to secure it down.
Steelflex brake hoses. The biggest advantage of this type of brake hose is not
better performance, at least not for this bike, but better resistance to aging.
They will not be attacked by sunlight like the rubber ones. The looks are
far superior of course. The reason why the Steeflex hoses do not help
much for better braking is the fact that the ATÉ swing type calipers
are a bit shape unstable. Try this: take a caliper off the fork, stick
a piece of metal between the brakepads ( like the disk-rotor )
and squeeze the brake handle... hard. Notice it deforms?
BRAKE PIPE CLAMPS
Next the on - fork brake pipe clamps, I didn't like 'm so they had
to go. This is the replacement, a stainless nut brazed to a
stainless strip, pipe fed straith through, looks clean and
fastens rocksolid with the help of a soft copper ring.
Neat and simple brake pipe clamp, no more rust. You can also go for longer
brake hoses connecting to the calipers directly, leaving the metal
pipes out. I thought about that option too late ...
ADJUSTING THE CALIPERS
Normally the adjustment routine for the excenter bolt in somewhat fiddly.
What I did, to make it a clean and easy job, was this.
End cap.....drilled 8mm
M8 bolt into excenter, use some studlock to make them a solid pair
No more fiddling around to adjust the caliper, the bolt could be a little shorter perhaps
Normally the excenter pin is pushed upwards by means of the spring, and so pushes the
caliper upwards against the upper fork finger, making the swing action somewhat stiff.
With the help of the '' outboard bolt " the caliper can now be vertically centered
to make it swing totally free of rubbing. Note, there is a small M3 bolt in
the now treaded bleedscrew. no more need for a rubber cap.
MAKE SURE THE METAL BRAKE PIPE IS BENT IN THE PROPER SHAPE.
IF NOT, IT CAN WORK AS A SPRING LOAD ON THE CALIPER'S
SWING ACTION, AND CAUSE ABNORMAL PAD WEAR.
Continue to Brakes, part three
Back to BMW R 90/6 maintenance index / Home
BR2 rev 2021
A A R D V A N H O L L A N D . C O M