Brakes rub when turning?

[Stoney]

Hi everyone, I've pretty much run out of ideas with this!

I'm riding a 78' sohc cb750k7, and since I first got it the brakes have rubbed when turning left.
It's not enough to notice when riding (sometimes a little squeal), but it's impossible to wheel around when turning left.
In an total overhaul of the brake system I have replaced:

The entire master cylinder
Brake caliper piston
Piston seal
Pads

It's running braided hoses which I have already checked for trapping/pinching.

I have also bled the system out a few times and fiddled with the caliper arm to get the right clearance.
Removing the small nylon ring that usually sits behind the brake pad has helped slightly, but the squealing obviously got worse.

I'm starting to suspect bad wheel alignment or the caliper arm being faulty, but thought I'd just get everyone's 2 cents before I get stuck into it this weekend as the forum has been an amazing help so far.

Thanks in advance!

 

[dirtdigger]

Check make sure wheel bearings are good and you dont have one side that is getting a little loose. Also if the forks are getting some play internally you can get some excessive twisting in the front end which can put a bind between the rotor and the caliper.

 

[Stoney]

Thanks man, will check the bearings this weekend. Also it looks like there is definitely some internal play in the forks, is this a case of adjustment or replacement? Probably due some new seals and fork oil anyway! Thanks

 

[Trionda]

<<Removing the small nylon ring that usually sits behind the brake pad has helped slightly>>

Is this a standard practice, and are there any downsides to doing this?
I am experiencing binding, and the wheel will barely spin on its own. I am in the beginning stages of troubleshooting, and will make any necessary recommended adjustments.

(My Back Story: I installed a new piston assembly in the original master cylinder, cleaned the caliper, checked all bores for wear, installed braided SS lines, and new fluid.
The front brakes work (a little scary, though), and I only just discovered that the wheel won't turn easily when I jacked the front end off the ground to rebuild the forks, and make some changes to the front end.

When the brake caliper is removed the wheel spins freely, and the wheel bearings seem fine, and in no way notchy. Brake pressure through the lever is extremely tight, as if it's over pressurizing and pre-loading the pads onto the rotor, and the lever comes in very little from resting position, even on a hard pull.)

 

[tommycarl]

As long as you're looking for potential causes, be sure to take a very close look at the groove for the caliper piston seal... if it didn't get cleaned out well enough it could be causing binding when the piston is trying to retract.

 

[Trionda]

As long as you're looking for potential causes, be sure to take a very close look at the groove for the caliper piston seal... if it didn't get cleaned out well enough it could be causing binding when the piston is trying to retract.

That's good info.
That may well be the issue, then.
I thought of getting one of those SS pistons from Ebay for 60 bucks or so, as well as new seals.
I'll probably do a rebuild, and perhaps if it won't behave right afterwards, I'll bite the bullet and spring for a new master cylinder assembly.
Thanks for the advice.

 

[tommycarl]

If you think it might be the master cylinder that won't let the caliper piston return, try loosening the bleeder screw at the caliper... if the pressure is relieved and the piston retracts, I'd say the master cylinder is more likely the culprit, not the caliper.

If you think it's the master cylinder, the first thing I'd do is clean the small holes you see inside when you remove the reservoir cap.

 

[Trionda]

Again, I cannot thank you enough for all the helpful advice you're providing. When you mention loosening the bleeder screw, is that all I should do, or should I pull in on the brake lever, and then re-seal the bleeder, or simply crack it open and see if it retracts?

I fully bled the master cylinder, and replaced the faulty piston, ran loads of new fluid through it, and cleaned out all orifices with compressed air. I still remain unconvinced that there may not be an issue in the M/C, itself, but I am hopeful that it may just be a lazy, dirty caliper.
The M/C piston bore was very clean, and unscored (I was surprised) after 40 years, but I had some issues fitting that last rubber grommet in place, and getting it to fully seal.

Again-thank you.
Cheers-

If you think it might be the master cylinder that won't let the caliper piston return, try loosening the bleeder screw at the caliper... if the pressure is relieved and the piston retracts, I'd say the master cylinder is more likely the culprit, not the caliper.

If you think it's the master cylinder, the first thing I'd do is clean the small holes you see inside when you remove the reservoir cap.

 

[tommycarl]

If you just loosen the bleeder screw any pressure on the piston should be relieved... if the master cylinder isn't letting the fluid return, then it will just escape through the bleeder screw. Just crack it open and see what happens.

I've also had a problem... just once... with the brake line not allowing fluid to return. Was a stock brake line and the one closest to the caliper. I never did know what was wrong with the line, but I replaced it and all was good after that. Sorry I can't tell you more about that one, but I'm sure I'll never know exactly what the issue was.

 

[Trionda]

I just completed our little experiment, and the brake pad only sucked in .002".
With that information, and the information you've provided, I have to say the brake piston/caliper assembly is suspect, as that's less than a sheet of paper's width.
I have just ordered a new Stainless Steel piston from an Ebay vendor in Washington State who specializes in US-made 304 grade pistons. I also ordered the piston seal, and plan to do a thorough cleaning of all grooves to ensure a good fit, and correct retraction.
Thank you again, for all of your help. This is the first site I've stumbled upon for CB750s that does not chastise it's members for asking mundane questions.

If you just loosen the bleeder screw any pressure on the piston should be relieved... if the master cylinder isn't letting the fluid return, then it will just escape through the bleeder screw. Just crack it open and see what happens.

I've also had a problem... just once... with the brake line not allowing fluid to return. Was a stock brake line and the one closest to the caliper. I never did know what was wrong with the line, but I replaced it and all was good after that. Sorry I can't tell you more about that one, but I'm sure I'll never know exactly what the issue was.

 

[tommycarl]

Okay, it sucked in .002"... but what happened with the wheel? Did it turn easier or not? I really have no idea how MUCH it should retract, but if .002" retraction gives clearance, what more is needed? Now I have to search the web to see if I can find the answer... dang! Anybody else know how far the piston should retract??

It certainly won't hurt to put in a new piston, but unless I saw something suspect on the existing one, I's be taking out the seal and thoroughly cleaning the groove, Gunk gets in there and can be hard to get out... and I've had more than one that I fixed with just a cleaning. I suppose it could be something else including a damaged or deformed piston, but cleaning is cheap. You naturally don't want to scratch up the groove, so be a little careful.

I'm guessing that you'll know pretty quickly whether it's dirt buildup or not just by seeing what's under there... but if that's the problem, you'll be riding safely again in a couple of hours or less. If you want to wait for the new piston that's certainly fine by me, but if you clean the groove AND put in a new piston, you'll never know what fixed it... and then we won't know what to tell the next guy with the problem.

 

[tommycarl]

So I did a search and found some pretty interesting stuff about brake calipers, piston movement , and seal design. I scanned through it pretty quickly, but here's what I think applies in this case:

1. The seal is actually what retracts the piston by design... really? wow, I didn't have a clue. I always thought it was rotors that aren't really perfectly flat along with maybe some wobble/movement of the wheel due to bearing end play and that type of thing. Anyway, if it's the seal and groove design, no wonder dirt buildup in the groove can cause problems.

2. In the illustrations shown in the link, piston movement is something around .2mm (.008")... more than the .002" you measured, but certainly not what I'd call significant, and likely influenced by the methods used in the experiment, both for moving the piston and tools for measurement.

3. Keep in mind that this was for a car... and who knows how much difference there is between the parts used and a CB750?

I'm kinda sticking to my suggestion that the groove be cleaned, assuming of course that the '002" you found didn't sufficiently free up the wheel.

Anyway, here's the link in case you want to read about it: http://www.simulia.com/download/sol...references/brakes_analytical_auc03_delphi.pdf

 

[Trionda]

The .02 DID free up spinning just a hair, but not enough to make the wheel spin freely.
The rotor is in great shape, and has a .004" runout by my guage, so is not suspect.
You may well be right about it just being the groove, and I will report back once parts come in. Though as you say, we may not have a definitive one-solution-remedy for "the next guy", I have found that these things are typically caused by a combination of more than one tired part.
The new piston will be a welcome addition, and if the original piston is not too far gone, it'll make a good spare if ever in a bind (brake humor there...)
Thanks-

So I did a search and found some pretty interesting stuff about brake calipers, piston movement , and seal design. I scanned through it pretty quickly, but here's what I think applies in this case:

1. The seal is actually what retracts the piston by design... really? wow, I didn't have a clue. I always thought it was rotors that aren't really perfectly flat along with maybe some wobble/movement of the wheel due to bearing end play and that type of thing. Anyway, if it's the seal and groove design, no wonder dirt buildup in the groove can cause problems.

2. In the illustrations shown in the link, piston movement is something around .2mm (.008")... more than the .002" you measured, but certainly not what I'd call significant, and likely influenced by the methods used in the experiment, both for moving the piston and tools for measurement.

3. Keep in mind that this was for a car... and who knows how much difference there is between the parts used and a CB750?

I'm kinda sticking to my suggestion that the groove be cleaned, assuming of course that the '002" you found didn't sufficiently free up the wheel.

Anyway, here's the link in case you want to read about it: http://www.simulia.com/download/sol...references/brakes_analytical_auc03_delphi.pdf