|Mushy Front Brake?
Try Back Bleeding
Thanks to Kurt "PizzaMan" Mirtsching
|Tired of a front brake lever that pulls all the way to the grip? No
amount of conventional bleeding solving the problem?
Back-bleeding is a solution used by auto shops around the
world. Here's how it works.
|Remove the front brake system in it's entirety from the bike, intact.
Unbolt the caliper from the fork, the master cylinder/lever assembly
from the handlebar, and unclip the hose. Don't crack the bleeder or
the master cylinder cap, keep the system shut. Then arrange the
system so that the bleeder nipple is lowest, the part where the brake
hose leaves the caliper is a bit higher, the hose is sloped uphill to the
lever, the spot on the master cylinder where the hose connects is
higher still, and the spot on the inside of the master cylinder where
the juice goes in is highest. I used my vise on my bench and a series
of stacked things next to my bench like cinder blocks, chairs, blocks
of wood and what not. Keep in mind the nasty corrosive nature of
brake fluid when choosing things to put under your brake system.
Get a great big monojet syringe. Agricultural supply places and feed
stores have them. I have one that holds 300cc, which is about five
times what your brake system holds. You want to connect this to
your bleeder nipple, so get clear aquarium tubing or clear PVC tubing
at the hardware store and whatever connectors you need to connect
the syringe to the nipple.
Fill the syringe with fresh brake fluid, and fill the connected hoses
with fluid too. Attach the hoses to the bleeder nipple. Get ALL the
bubbles out of the tubing and the syringe. There can be air at the
plunger end of the syringe as long as you hold it plunger-up and
don't squirt so far that it enters the hose.
Once all the bubbles are out of the tubing and syringe, crack the
bleeder nipple and gently plunge new fluid into the system. It should
push all the old fluid, and your bubbles, out through the master
cylinder. The master cylinder resevoir will over flow, so have a helper
there to suck it down with another syringe, or have a bunch of rags
there to catch the overflow, or have stuff under the master cylinder
that you don't care about.
Some calipers have chambers that can have high internal cavities that
will hold air the way air will stay in an inverted glass underwater. The
bleeder valve SHOULD be the highest point on the inside of a caliper
cavity, but it sometimes isn't. So, as you are plunging the fresh fluid
into the caliper, without letting the hose come off the nipple, roll and
rotate the caliper around so that any internal trapped bubbles can
make their way to the bleeder valve and on out through the brake
hose and out the master cylinder resevoir.
If you run out of juice in the big supply syringe while you're doing this,
just shut the bleeder valve, pull the plunger out, add more juice to the
supply syringe and re-insert the plunger. If you do this right, no air
will enter the hose at the bottom of the syringe, and you can continue
to pump juice through the system.
After you've pumped a bunch of fluid through, shut the bleeder valve
and button up the master resevoir. Clean everything up and re-install
on the bike. If you did it right, and this doesn't do the trick, then you
have some other problem, such as a faulty hose that's expanding, a
warped disc, or bad rubber in your master cylinder or caliper.
Keep in mind that fresh pads sometimes need to "take a set" or break
in. They don't exactly mate with the disc and have to rub off just a bit
of material. Riding 100 yards with your brake held on can accomplish
this. Brand new pads that need to take a set feel just like air in the