Topic: CB350F - Master Cylinder Rebuild

[Nezhac]

Hey all,

So here goes the story:

About two months ago I had a front brake seize, nothing special, read the FAQ, took apart the pivot and caliper, both were seized pretty good, clened it all out, dow corning grease and back together, marvelous.
However, I had noticed the wheel didn't really spin free, deciding not to worry about it and see it evolve

Now, two months later, my dad acquired a 750 k5 in good general condition, i tried just spinning the front wheel and there is MUCH less drag than mine. Just 2 days ago, the bike almost seized again, i had to switch to the rear brake, pulling off from a stadstill had me slipping the clutch more than usual. I decided to rebuild the master cylinder, in my reasoning it was the last possible culprit in the brake system for a stiff front wheel

I have a mastery cylinder rebuild kit, here's a picture with the brand and the grease I got with it:
http://www.flickr.com/photos/nezhac/4395810076/

now here are my questions:

• What is this grease for? I know to coat the inside with brake fluid before assembly, but should the grease go anywhere?
• secondly, the second hole in the brake fluid reservoir is blocked off, I think I remember reading about it here that it was normal, but can't find that thread anymore so not sure. there doesn't seem to be an opening for it in the cylinder wall.
• thirldy, how clean does it need to be before re-assembly, it's hard to find proddy things that get in there without scoring the walls. the hardest stuff to get out is at the bottom of the cylinder, looking in from the brake handle side.
  

I should also add that once the mastery cylinder was off, and the caliper still on, the wheel spun freely again, so I assume the MC is the problem, am I right?

Thanks for any ideas, helps or comments 

[hoodellyhoo]

The only thing I would use the grease for would be to lube the brake handle pivot.

What exactly do you mean by the "second" hole in the mc? Do you mean the big hole or the little one?

Get that sucker as clean as you can. I bought a set of cheap HF dental pics for my last MC re build and they really helped with the rust and crud around the circlip groove. I did the best I could but there was still a little really stuck-on stuff at the very bottom that I just left.

When taking your mc apart, compare your original parts to the one in your rebuild kit and make sure they are all the same.

[Nezhac]

I mean the two holes in the reservoir itself. On this picture its the hole on the left:
http://www.flickr.com/photos/nezhac/4396375862/

[Nikkisixx]

Both holes look like they are the same size, and at the top they are.  In the bottom of the "second hole" is a smaller hole.  That hole is very small - about 0.015 inch or about 0.381 millimeters.  It should also be open, but at first glance it may appear closed.  A small pin or wire is usefull to gently clean that small hole.

[camelman]

I had the same thing happen on my CB400F and CB350F... same brake system.  One the 400F, the nylon washer had slipped out of place between the piston and the brake pad, and it wedged the brake pad out too far.  It ended up locking the front wheel on me and I took a dive.  One my 350F, I just had to pull the caliper apart and clean it out really well again.  It might have had a bit of air in there too, although I think the main issue was getting the rest of the crud off the piston.

Neither bike had an issue with the master cylinder not working correctly.  No matter what, I think it is hard for that master to not allow pressure to bleed out from the system.

Good luck.

Camelman

[wdhewson]

Hi:

I've used a minor amount of this silicone grease to lube the rubber seals and cups in the master and slave cylinders.  It's very inert stuff chemically.  If you assemble with brake fluid and leave a then film before getting it back on the bike it will absorb water/humidity and there will be little concentration of the anti-corrosion additive there to prevent rust etc.  Use the Silicone grease, but don't over goop things.

One of the holes you mentioned feeds the cylinder of the m/c from the reservoir, and the other hole releases the pressure developed by applying the brake.  Make sure that this pressure relief hole is not plugged with debris, make sure that your piston retreats entirely, and this retreat is not inhibited by debris or weak spring, and make sure that you've got a couple of millimeters of clearance between the heel of the lever and the piston of the m/c.

When rebuilding slave and master cylinders, do everthing you can to minimized dust and dirt.  They will tolerate some solids, but it is best to be surgically clean in this work.

END

[Nezhac]

Alright!

Thanks for the tips and stuff guys, got the MC back together on the bike and bled this afternoon. I'm quite sure it was the return hole, as it was completely clogged up when I took it apart, it take a fair amount of insistent cleaning with a mix of carb cleaner and de-rusting compound, and then finding some wire small and stiff enough to get in there to make sure it was clear was a bummer, but victory prevailed!
http://www.flickr.com/photos/nezhac/4414893532/in/set-72157623449394405/

And I put a tiny bit of that grease I got on the rubber parts, thanks for the tip.

So far, it rolls much much easier than before, like my dads 750 more or less, we'll see how it evolves.

Does anyone have a tightening torque for the bolt that goes through the brake line into the MC? I did it by hand to about 25Nm. enough so that it doesn't leak after a couple of hours