Greenberg Brake Bleeding Method
By Steph Greenberg
Most of the time, HD's come out of the factory with brakes that feel
"soft". This is attributed to expansion of the rubber hoses used for
the brakes, and people change to stainless lines for that reason. In
all four Harleys we've gotten, every one of them came complete with
air in the brake system. I suspect that the reason people feel an
immediate difference in the firmness of the brakes with stainless
lines is that it's the first time since they've owned the bikes that
the system has been bled thoroughly. Although I think the difference
between the stainless and stock brake lines is less profound than
people popularly think, I personally prefer braided lines because I
think the stock rubber lines are more susceptible to damage and
catastrophic failure over time. The stainless braided armor
protects the lines from external damage and aids in restraining
expansion over time.
If this is the very first time you're thoroughly bleeding the system
or you've changed brake lines this is what I do:
Before you even think of starting this, do yourself a favor and buy no
less than *two* bottles of DOT-5 brake fluid. You'll probably waste a
lot of fluid in this process, and if you end up with some extra brake
fluid, you'll use it eventually.
Pump up fluid *from* the caliper to master cylinder. I do this after
bleeding the brakes (conventionally) as thoroughly as I can with the
Mighty Vac. I then configure the Mighty Vac to pump. You have to run a
short hose to the receiver cup, fill the cup with brake fluid, and
pump air in from the other side of the receiver cup by using the
"exhaust" side of the Mighty Vac as your pump.
Get the fluid to the end of the hose so there's no bubble from the
line. Then attach the rig to the bleeder valve and pump. You might
have to use a worm-gear type hose clamp to tightly affix the tube to
the bleeder valve. Pump the fluid up through the line. You'll be
surprised by the fact that air bubbles appear immediately in the
master cylinder. Then once you get that done, you return the Mighty
Vac to normal. I've also loosened the banjo bolt at the front master
cylinder. This can and probably *will* be messy. Cover everything you
can with plastic trash bags if you don't want to be wiping brake fluid
every part of your bike from the handlebars down.
For regular bleeding, which I do at least twice a year or before I'm
taking a trip where I expect to go above 5,000 feet (living 100 feet
above sea level):
Motorcycle brakes seem to be plagued by air in the system that you
can't get out through conventional bleeding. That's because air gets
trapped in the caliper pistons themselves. There's only one way to get
that air out, and that is to compress the pistons in all the way. The
best way to do that is with full access to the caliper, with the brake
pads out, either by removing the caliper or removing the wheel.
I prefer removing the wheel because it's easy and fast. But on some
bikes, it might be easier to remove the caliper. Just carve a note on
your forehead that reminds you to use a torque wrench and torque the
caliper back on to spec. I don't want to hear about someone landing
themselves in the hospital or worse because they took the caliper off
and didn't tighten the bolts when they put it back on.
Once the calipers are off and the pads are out, you put a flat bladed
tool of any type (big screwdriver, or those automotive flat bladed
brake tools) between the pistons for the 4 pot brakes or between the
piston and the passive pad on single pot brakes, and you pump the
brake until the piston(s) is(are) firm against the tool. Then you
firmly push the pistons back into the caliper while opening the bleed
valve. You should see bubbles come out of the bleed valve. Even if
your bike just came off the delivery truck from the factory. I do this
about 3 times before I declare the caliper air free.
It's important to remember not to open the bleed valve until you have
some pressure on the pistons. If you open it sooner, you risk sucking
air *into* the caliper or line.
The Quick and Dirty method of bleeding the calipers:
If I'm in a hurry or find I've got a problem on the road, I have a
quick and dirty method of bleeding the calipers. I stick the flat
bladed implement (the automotive brake tool still seems to work best)
between the brake pad and the disk rotor, pump up the system with the
brake pedal or lever, and force the pistons in on each side (on my 4
pot brakes). This is effective enough but it has some disadvantages
unless done very carefully:
1. You can actually damage the rotor or even bend it
2. You can damage the brake pads
3. It's really hard to compress all 4 pistons on the 4 pot brakes at
the same time. When you're pushing two of them in, it's pressurizing
the other two pistons in the caliper. Probably not a problem, but it
makes getting the blade in the compressed set that much more difficult.
4. You can't push the piston in all the way, so there may be small
amounts of air trapped inside the bores.
5. The caliper is at a slant where the bleed valve isn't at the very
top. This can trap bubbles at the highest point in the caliper, a
problem particularly true of rear brakes where the caliper is way off
the optimum for bleeding.
6. Getting the tool under the pad is a challenge in itself. The first
time you try this method, it will probably be accompanied by some
swearing and a need for Band-Aids for the scraped knuckles you get
when the tool slips and you bang them on spokes or parts of cast
wheels you couldn't have imagined as being sharp enough to draw blood.
Now some people ask, "If I've bleed the system, how is air getting
back in there every few months?" Well, several discussions of this
have yielded the theory that the DOT-5 brake fluid we use in Harleys
and other bikes allows nitrogen to dissolve into the fluid, and when
you go up in altitude, the bubbles enlarge and form a pool of bubbles
that becomes an "air gap". The bubbles also are supposedly driven out
of solution by the heat generated by the brakes over time. Whatever it
is, air bubbles mysteriously appear in the system over time, and
aren't a real problem until you change altitude or allow them to
accumulate for too long.
Copyright 2002 Stephen Greenberg.
Permission granted for use on metrohog.ia.net only.
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