RE: Brake Nightmare
 

I should have thought of this before.

When the caliper is seized and the wheel won't turn, try to push the caliper back by hand and see if it relieves. If not, then crack the bleeder screw and see if that relieves it. If it does, you've DEFINITELY got the fluid captured between the master and the caliper. Go to the master and find out what's up, largely as described above. That proves the caliper pistons aren't stuck.

RE: Brake Nightmare
 

Dad,
OK, I have now done the following:
Replaced the pins, ( they were only a little coroded, but only a few quid so what the hell?)
Rebuilt master cylinder.

I am not left with much now am I? Just to make sure, I have fitted the pressure cups within the cylinder both pressurising the hose outlet, hope this is right. How exactly does the master cylinder allow fluid to return from the calipers? Have I missed something in the cylinder, as I couldnt see a return flow path. Despite having worked on hydraulics, I cant actually fathom how this master cylinder actually works. Why the helix built within?

Lastly, I am only left with the following ideas:

Could it be the braided hoses?

I might have to try buying new seals again, and reftitting the calipers again.

The pistons do move, but I do need to apply the force of a small G clamp to get them to do so, I cannot physically get them moving with hand pressure, even with bleed nipples open. But the G clamp isnt using much force, it must be just above what I can manage and no more. As no oil can reach the dust seals, perhaps I need more silicon grease here to allow less friction build up? A straw clutch I know but for goodness sake!!!

Really fed up now.

Hope you can add some kind of insight

Ara

RE: Brake Nightmare
 

I don't envy your frustration.

The master picks up fluid simply. The piston moves past a drilled port in the side of the bore that opens it to the reservoir. The fluid gravity feeds to the bore and will fill as needed or return any excess it has to the reservoir. A simple open line between them. As you depress the pedal, the piston moves past that open port and captures the fluid in the system. Depressing further acts on the caliper pistons, forcing them outwards against the pads, clamping on the rotor. That's really about all that there is to it.

With those basic functions in mind, you can see what would happen if the master was not returning all of the way. It would capture the fluid in the system and a small amount of expansion from heat will result in captured pressure, putting force on the pads, applying brakes, generating more heat, therefore more expansion, therefore more brakes..... in a vicious circle that ends with a seized brake.

The other main source of trouble is a seized caliper piston. The problems that creates are fairly obvious and something you've apparently considered. To prove that system out to yourself, if the pistons slide easily in the bore without the seals installed, no mechanical restriction to their movement, then they're good. The seals will drag and are supposed to. They seal but also act as the piston's return spring. The last bit of motion at the caliper after pressure has started to build is the seal being pressed hard against the bore, no longer sliding but flexing that last miniscule amount. On release of the brake pedal, therefore the pressure, that seal flexes back that same miniscule amount, moving the piston back slightly, assuring no hard brake apply. A small amount of drag but no apply.

One thing that's true of these is they do tend to drag the rear brake a little excessive. It's in the design and I think the result of a relatively small cross section caliper piston "O" ring, not flexing or returning much. Strictly an observation I made that is not based in any true analysis, just that it struck me as smaller than what I was used to seeing in caliper piston seals and I thought the drag was a little excessive as well. That's a hard number to define. Afterall, what's excessive?

The best way to tell if it's a seizing problem is to assemble it and ride it. If the fluid's captured the brake will apply harder as it warms. Ride a few miles, stop, put it on the centerstand and see if the brake is dragging harder. Also see of the rotor's getting hot. If not, try a few more miles and do the same. Repeat until you're satisfied it's not a problem and the brake drag hasn't changed from stop to stop. If it's really dragging it will get the rotor COOKING HOT in a very few miles. If you care to prove that to yourself, try dragging the rear brake on purpose for a short time and then feel the rotor. It doesn't take much to get it hot.

One final consideration is if the interior of the brake hose has swelled to close and started to act like a check valve, expanding under pressure to allow fluid through but closing back up and restricting the free return of the fluid. That's a theoretical problem that I've heard of in brake systems, most likely caused from a fluid not compatible with the rubber, but also something I've never encountered in any brake system, car, bike, or otherwise. I tend to think of it as one of those urban legends unless someone filled the system with motor oil or some totally incompatible fluid. Not the first thing I'd expect to find but at least theoretically possible. There just isn't much else. It's a pretty simple system and the basics covered are pretty much all there is to it.

Are you possibly a bit oversensitive to the feel due to your concerns after having it apart? I know I can get that way sometimes.;) Good luck and let us know what you've found.

RE: Brake Nightmare
 

dad,
No, not over sensitive, Honest!!. The rebuild went fine, and passed its MOT no issues at all, front brake, slightly draggy, no more. Over the next few days, the front brake got stiffer, (I suspected as more of the air bled out under its own steam) and then the front brake got to the point once again, that I cannot move the wheel by hand on its centre stand. This is when i went back to the forum and read about the master cylinder theory, and fitted a repair kit, (after cleaning it out etc). What my thought was is that the pistons were so tight (but only within their dust seals, as fitting them without dust seals, but with pressure seals was easy to do) that the dust seals were holding the pistons out instead of allowing a little 'give' to go back in. I am trying again tonight, and will COVER the damned pistons in silicon grease (only kidding) and see what transpires. Just for info, did exactly the same procedure (bar the master) on the rear,and it spins completely freely by hand, only the chain actually slows it, which was what I was expecting. Will let you know my anguish on Monday/weekend.

Thanks again!

Ara

RE: Brake Nightmare
 

Ara

What was the final resolution? I have a similar problem. I recently purchased a '93 CBR1000f, put about 500-600 miles on it uneventfully and determined that some maintenance issues needed dealt with. After the mechanic was finished, I took the bike for a test ride and the rear caliper engaged after 4-5 miles. Seems if the bike sets it will release the pressure though. Mechanic was not in the brake system although had front calipers off forks.

LDGF

RE: Brake Nightmare
 

Hi Aracanth.

New to this forum and this is my first post.

I've had my 89 cbr1000f-k for a few years now and it's showing 56000 miles on it. I too have had the same problems that you mention with the binding brakes etc.

I've rebuilt the calipers with new seals on more than one occasion and still had the problems you're getting now. Reading into your posts it's clear to see that it is a piston problem and not a master cylinder one as the pistons don't want to return even with the bleed nipple open. Again, I too had this same problem. The cure?

Take the pistons out, remove the dust seals. then put the pistons back in. You should then find that the pistons return with hand pressure. This then tells you it's a problem with the dust seals, as it was with mine. The groove that the dust seals sit in gets gummed up over the years, effectively pushing the dust seals against the pistons jamming them pretty good. I took my dust seals out and scraped the groove out of all it's crud. It was quite surprising how much stuff I got rid of. I was lucky enough to get hold of a dentists scraping tool to do this with. The seals then sat all the way into the groove alleviating pressure.

Now, no binding, although I overheated my discs on a previous rebuild so need a new set.

Don't be tempted to ride without the dust seals in. Although the pistons will return nicely you'll soon fill the caliper with crap making the fluid seals leak. I know someone who did just that;)

Another option? And one i'll be doing soon.

These cbr's now all seem to have lunar miles on em, so a swap for younger, less worn calipers could be an answer. I'm sure some of you guys are aware on here, but just in case you're not - Early 600 Bandit calipers are the same, also rf600, some 600 hornets (i think) some cbr6's and vfr750's amongst others. I also took measurements off a mates Honda Dullville with brembo calipers on it. The mounting bolts on the front forks have the same spacing as my cbr so i'm thinking of trying a set of these to see if they fit. You never know.

Good luck with your brakes, and let me know how you get on.

RE: Brake Nightmare
 

I've heard about wheels that were chromed were now too thick, and the wheel bearings were binding in the bore. Could be the same with powder coating. I also read in Cycle World magazine about front brakes that were binding and it was caused by aftermarket brake levers. They were not allowing the master cylinder piston to recede all the way.

RE: Brake Nightmare
 

Hey just as a wierd theory. Could it be something in the DCBS servo that is causing the problem?

RE: Brake Nightmare
 

my 88 CBR1000F had dragging front calipers as well, I just took the pads/calipers off, and pumped the handle a coupl etimes, then locked it back with a zapstrap, and worked the pistons in and out with clamps until they wre pretty easy to move big welders's ViceGrips worked well, and I put some brake lubricant on it and did this about 50 times until they were pretty free... 5000 kms later they're still alright, they do drag a little, but not badly


On anothe note, my katana 750 has a spongy lever.. new SS braided brake lines, bled 1000 times, good calipers, havne't actually rebuilt the master cylinder, but what else could it be? the master cylinder seems ok

RE: Brake Nightmare
 

Hi Rx7.

I had a spongy lever on my CBR and used an old tip to cure it. Bleed the brakes as best you can then pull the lever towards the bar as far as it will go and tie it there with maybe some cable ties or something. Leave overnight and voila!

The other thing I do to get a decent lever is to again, bleed the brakes as best you can, then, with the resevoir cap off, start hitting the lever in with the palm of your hand as fast as you can. Just keep pumping the lever real quick and you will notice tiny air bubbles rising through the fluid in the res. Jobs a goodun. Good luck.