cb750 master brake cylinder assembly

I am pulling my hair out on this one. I have an old CB750 which has (sadly)
sat in a shed in my backyard for about 8 years. I sold a few motorcycles
this spring, basically all the working ones, hoping to pick up a Ducati that
an acquaintance was going to sell. Sale never happened, so here I am stuck
with no bike. Ahhhh, the old CB in the shed. So I spent some time this
summer getting it to a roadworthy state, in fact I've got the motor running
very well after a top-end tear-down.

Here's the problem: I cannot get the front brake cylinder to work. I
ordered a rebuild kit off eBay which looked identical to the parts I took
out. I assmebled as per the schematic shown on the Bike Bandit website:

http://www.bikebandit.com/houseofmotorcycles/1973-honda-motorcycle-cb750k3/o/m9234

The kit was all the parts inside the polygon noted as #13. For the two parts
of #2, the seal on the right slides over the end of the piston and seats on
a ridge at the left side of the thin part of the piston.

I also changed out the brake lines (replaced with braided steel) and the
caliper piston seal.

When I put it all back together and added some brake fluid, it didn't leak,
nor would it bleed. I put a piece of tubing onto the bleed nipple on the
caliper and dropped the end into a jar of brake fluid so I could see
bubbles, or lack thereof. The first pull on the lever pushed out a big
bubble, then I closed the bleed nipple and released the handle. When I
opened up the bleed nipple it sucked fluid up into the tube, there was a
vacuum in the system... so it appears no fluid was getting past the master
cylinder seals to replace the air that got pushed out. I removed the brake
line from the caliper and did a little test. I put my finger tightly over
the outlet and speezed the lever. Yup. Pressure. I then released the
pressure (keeping the lever pulled in) and put my finger back and released
the lever. Vacuum. Waited a few seconds thinking it'd pull some fluid or
air or something past the seals but no, it was still a vacuum after 10
seconds or so.

I purchased another master cylinder off that auction site thinking maybe
mine was hosed, but symptoms are identical. I tried using a syringe to make
sure I had fluid filling the cylinder, i.e. no air in the chamber.

Anybody have any ideas? Am I missing something fundamental? Did I maybe
get the wrong rebuild kit?

Dave

 

From right (where brake line attaches) to left (where brake lever pushes) I
put in:

small metal cup with holes in it, concave side facing right, convex side
fits inside spring

metal spring, big end towards right (with metal cup stuck in end).

seal fits over top of small end of spring, concave side faces right.

Piston with seal around shaft at left side of thin(ner) section of shaft
(about in the middle). Seal is like a round trough, concave portion facing
to the right.

Centering washer

Lock ring

Dust seal

retainer ring

 

There are two holes in the bottom of the brake fluid reservoir: one towards
the lever end which obviously goes straight through into the cylinder, and
another hole towards the output end of the cylinder which doesn't look at
first glance like it goes all the way through... it looks like it's drilled
partially through. maybe what I'm seeing is the metal clip you reference?
Or maybe the hole is drilled in such a way as to add a bend to diffuse the
flow so it doesn't squirt.

 

Did you actually compare the lengths of the old and new pistons?

Years ago when I had a CB250G5 I ordered a complete hydraulic service kit -
the local shop somehow contrived to obtain a kit with a piston 1.5mm longer
than it should have been (possibly a CB400F kit).

It took all manner of "thinking outside the box" to succeed in bleeding the
system and the front brake was binding all the time.

The way I bled it was to use a large syringe full of brake fluid on the
bleed nipple and slacken the top banjo to let the air out - it didn't occur
to me at first that was a symptom of a serious problem.

Hey - it was my first ever hydraulic brake!

 

Those 'sucker' pumps work well for bleeding brakes. Just put some
teflon tape around the threads of the nipple and don't screw it all
the way in. That seals the fitting enough to pull a vacuum so you
aren't just pulling air.

As you say,you may have more problems then that, but a good system to
bleed the lines means one less thing to worry about.

Rick