Topic: Master Cylinder Problems?

[LoopsAndLogic]

After rebuilding my master cylinder on my 1981 CB650C, I find that the master cylinder won't push any fluid.

I use my vacuum gauge at the caliper and no fluid will drain down through the hoses. Then I try pumping the system, and I get no pressure.

 I then take the master cylinder off and while it's in my hand I push down onto the brake lever and no fluid will even come out of the banjo opening.

 Before I installed the K&L rebuild kit, the bore was completely smooth with no visible nicks at all.

 The M/C is full of fluid and I was wondering if I need to have the top cap off when I'm priming the system??

Should some fluid come out into my hand when I squeeze the lever??

 Right now I'm at a stand still.  And the master cylinder is the dual caliper type.  Could I have the wrong K&L kit??

Thank you

[bryanj]

First wear protective gloves!!!!! VERY IMPORTANT

leave the master bolted to the bars
Fill the reservoir
remove the outlet banjo
put finger, protected by glove, over the outlet hole sealing the hole
(1)pull in and hold lever
(2)lift off finger and replace
(3)release lever
Repeat (1), (2) and (3) until fluid starts pumping then whilst holding in lever refit pipe and banjo

Might sound crazy but after years of practice believe me it works

[LoopsAndLogic]

Thanks! Will try that tomorrow or Wednesday.

I've never had to do this before. Could you explain what I am doing wrong?

[Hockers Choppers]

I'd check the rubber plunger in the m/c. I used a rebuild kit one time and the skirt on the plunger was too long and it sealed off the hole that the fluid goes thru from the reservoir. Do You still have the original one to compare it too?

[LoopsAndLogic]

Unfortunately I don't. What hole would you be referring too??

Cause I see 2 holes, but I don't know which hole is for the fluid to return.

 I will look at the plunger tonight and get back to you.

Thank you for all your help

[Hockers Choppers]

Also make sure that both holes are clear. The smaller one is very tiny, I don't know if a paper clip wire will go thru it.

[bryanj]

The problem is that with everything connected and the cylinder dry there is just too much air to move and it keeps compressing and expanding! When the master has starteed getting "wet" (for want of a better word) and moving fluid it will all eventually come out. All you are doing with my method is using your finger as a "Bleeder" and reducing the volume.

THE WARNING is cos brake fluid has been found to be one of the greatest causes of oil based dermatitis and if you work with spanners that just about finishes your career and yes i do know that for a fact only my cause was cleaning fluid in a parts washer!

[Hockers Choppers]

Thats why I went with dot5  and cleaned all the parts of my brake system very well and went to stainless steel brake lines. Stops like a modern bike now!!! I have a good source for lines if anyone wants them.

[kettlesd]

If the MC is OFF and in your hand then you should definitely have fluid if you squeeze the lever. Any chance your rubber piston cup is put on backwards? Its not hard to have it happen actually when slipping it over the piston flange - or even to have it turn inside-out. The wider end of the cup shoudl face downstream toward the lines. If it is backward, it would have the effect of trying to draw fluid INTO the MC when you squeeze the lever instead of out of it.

[dpen]

Another way is to bleed the m/cyl itself first.

Fill with fluid & then pull the lever in slowly & gradually

When an air bubble pops from the hole in the bottom of the reservoir STOP, let sit for 30 seconds & pull lever in again

You'll get three or four air bubbles each time

DON'T PULL THE LEVER IN ALL THE WAY or you'll have to pull the cylinder apart to reseat the piston seal

This method takes about 20 minutes  ('till theres no air bubbles) & then you bleed as normal from the caliper(s)

I've used this for eons on bikes/trucks/cars etc etc

[tmht]

If the MC is OFF and in your hand then you should definitely have fluid if you squeeze the lever. Any chance your rubber piston cup is put on backwards? Its not hard to have it happen actually when slipping it over the piston flange - or even to have it turn inside-out. The wider end of the cup shoudl face downstream toward the lines. If it is backward, it would have the effect of trying to draw fluid INTO the MC when you squeeze the lever instead of out of it.

You beat me to it Kettle. I just did this very same thing this week. Made me feel like a dumbass. Then it pissed me off because I had to deal with that stupid circlip all over again.