Front brake rubbing, but also not stopping well...

[Stoney]

Hi there!
I've just bought a bike that has been stood a while, it was hard to wheel around due to rubbing brakes and was not stopping well at all with hard application of the front brakes. I've changed the brake pads out and bled the system about 3 times. The piston appears to be moving freely when the brake lever is applied, so it doesn't appear to be seized. None of the hoses are trapped or kinked.

This is my first encounter with the cb750 brake system and I'm starting to wonder if I've got the adjustable arm that the caliper is mounted to set up right, I've tried to align it as-per the manual specifications but not had any joy.

Brakes are not my strong point and I apologize if there is already a really good thread on this forum for brake adjustment that I haven't spotted!

Thanks guys. Its very annoying to have an otherwise perfectly functioning bike off the road with one stupid problem!

 

[tommycarl]

I think the rubbing and not stopping well are likely different issues: one being a dirty caliper, another being a not fully bled system or a dirty or faulty master cylinder. Your description of the issues doesn't include quite enough details to pinpoint a single cause.

Regarding the rubbing: I'd say that either the master cylinder isn't relieving the fluid when you release the lever, or the caliper piston is dragging and won't retract completely. To check the caliper piston, I suggest that you simply loosen the bleed screw at the caliper and see if the dragging goes away. If it does, that kinda points to the master cylinder as the culprit. If it still drags, the caliper probably needs cleaned, especially the groove that holds the seal (inside the caliper housing). To clean that grove, remove the caliper and pump out the piston. then disconnect the brake line so you can get to the inside of the caliper housing. Be sure to clean all the debris in the groove, and get down into the corners of the groove.

The confusing part to me is that you say the piston seems to move freely in the caliper... kinda means the caliper isn't the issue. However, just because it moves freely outward, doesn't mean it retracts freely, so a better understanding of that will be helpful.

As for the issue of not stopping well, it would be helpful to know if you get decent pressure when you squeeze the brake lever, or if you have to pump it up. If it needs pumped up to get pressure, it could just be that the system still isn't bled well. If you get good pressure with one squeeze of the lever, your bleeding was probably okay.

For now, could you please advise on the results from loosening the bleed screw, and whether or not you have to pump the lever to get pressure? Also, please confirm whether or not you have a stock master cylinder and caliper.

 

[Stoney]

Hi Tommycarl, thanks for your reply. I'll try loosening the bleed screw when I get home and see if that narrows things down as you suggested, I'll also check whether the piston is retracting properly while I'm at it. With regards to the lever pressure, it certainly doesn't need pumping to get pressure and does not pull all the way back to the handlebar grip, but does seem a little spongy. As you said, it's most probably a collection of different issues rather than one, so I'll bleed it again too.
One other thing I meant to check, is the brake pad surface on the caliper meant to sit all the way in so that its flat with the inner edge of the caliper itself? It currently sits about 4mm out from the caliper, I'm wondering if I should loosen off the bleed screw and squeeze the pad in a bit further before fitting.

Thanks again for your feedback.

 

[tommycarl]

The pad should be right against the rotor. If you push the pad in further you'll just be pushing it back out when you apply the brake... and that will require some pumping the first time most likely just to move enough fluid to push it a longer distance. When it backs off, there won't be any noticeable gap between the pad and rotor.

It sounds like you have the system bled pretty well since you can't pull the lever all the way to the grip. There could still be a bit of air at the joint on the forks I suppose. You might try bleeding at that banjo fitting by loosening the bolt just slightly and squeezing the brake one time to see what happens. You should naturally tighten the bolt again before you release the lever. Before loosening it, turn the handlebars all the way to the left to get the RH side of the joint at it's highest point so if there's any air there. it will be at the top and at the higher banjo bolt.

I suppose there's also a chance that your hoses are "soft" and you could be the reason your brakes feel spongy. I've never had that happen on any of my bikes, but have heard that it can be an issue. I've also heard that replacing stock type hoses with braided hose makes a big difference... again, not something I've done.

Please confirm that if you squeeze the brake lever as far as it will go that it doesn't eventually get to the grip if you keep squeezing it. The basic question is "does the system hold pressure once you squeeze the lever"?

And please confirm whether or not you have a stock caliper and master cylinder. My reason for asking is to be sure that the master cylinder hasn't been changed to one that might be too small, or if the caliper might have been changed to one (or two) too big for the stock master cylinder. If the master cylinder doesn't move enough fluid for the caliper(s), it can feel spongy and won't stop well.

 

[Stoney]

Thanks for the continuing advice! I loosened the bleed screw as you suggested and it made little difference to the rubbing, I'm going to try giving the caliper a thorough clean next. Is there an easy method for dismantling the caliper? I've heard that running a compressor into the open bleed can pop it out.

Also I just double checked the lever and with a bit more force it IS possible to pull it all the way to the handlebar, so probably a pressure issue too! Joy.
The caliper and master cylinder are both stock, but the previous owner has changed out the cables for braided ones. I've noticed that the rubbing gets worse with full left lock on the steering too, not sure if that has anything to do with anything! Any tips for overhauling the master cylinder too?

Slowly building up a list of sunday jobs! Thanks again.

 

[tommycarl]

Whenever I have a piston that doesn't want to come out I just use the brake lever to push it out. You've said that the piston moves relatively freely, so you might be able to pus it out with compressed air, but I've actually been able to use air on a tough piston. For really hard cases I've heard that you can use a grease gun to pump them out... I've never had one that bad. Just a warning about the compressed air... if the air will push it out, it could come out quickly, so watch where you have it aimed!

The way you describe brake lever travel, it sure sounds like the master cylinder is doing it's job, at least in providing pressure to push the piston from the caliper. What I was really looking for with my question about continuing to squeeze the lever was if the seals in the master cylinder might be letting fluid pass rather than maintain a constant pressure. As hard as it sounds like you're squeezing to get it to the grip, it stills seems that it could be a system bleeding issue. Sometimes it's really hard to get the air out.

However, since the piston doesn't retract far enough to eliminate the rubbing, it could be that the small return hole in the master cylinder is blocked. At this point I'm leaning toward the caliper and bleeding being the most likely issues. I certainly hope it isn't both the caliper and master cylinder.

As for rebuilding the master cylinder, the most important part in my opinion is that you have snap ring pliers that will reach the snap ring. Once the snap ring that holds the plunger in is removed, the job os relatively easy... unless the plunger just doesn't want to come out. Actually, there's another place for that air pressure you mentioned. I've pushed the tip of my blow gun into the end of the master cylinder to push out the plunger. I find that it can help to put a rag over the hole to get a better seal at the gun tip, so keep that in mind if you try it... just one layer of rag over the hole, don't fold it a few times.

Also. don't try pushing the plunger out with something like a dowel or nail, or something like that. There's a small "dome-shaped" with several small holes around it and you can destroy that piece. If you have a rebuild kit with that piece in it, it may not matter to you, but just a word of caution if you need to reuse the pieces.

I won't go into more detail about the rebuild, but if you have no clue what you're getting into or how to take it apart, I can give you some more information based on issues I've had.

 

[tommycarl]

I checked youtube for a master cylinder rebuild video... here you go: http://www.youtube.com/watch?v=2W24asyNujs

He doesn't go into enough detail for me regarding the direction the seals oN the plunger go... PAY VERY CLOSE ATTENTION to how the seals are positioned when you take it apart... that assumes, of course, that the previous rebuilder put it together correctly.

I also want to warn you about pulling off the rubber boot like he shows... if you're planning on re-using the boot, be careful pulling on it because it can tear pretty easily.

 

[Stoney]

I can't thank you enough for all this information. I think you may of been right on the money with the caliper and bleeding being key issues.
I managed to get the piston out of the caliper very easily, I gave the inside of the caliper and the piston a good clean and noticed that there is some pretty bad pitting and a few scratches on the piston. I re-assembled and bled the system, loosening the banjo bolts as you directed. I'm now getting much better pressure on the lever and better response from the brakes, but also the rubbing has gotten a lot worse, almost to the point where it is impossible to wheel the bike around. The master cylinder is definitely sucking fluid back up (I poured some fluid into the open caliper when the piston was out and operated the lever). I've noticed that the rubbing is much worse when I turn to the left, but none of the hoses are catching on anything. I'm close to just getting it to the garage at this point!

 

[tommycarl]

Well I'm glad that at least you think you may some progress, but it sounds like it was mostly with getting the system bled better.

I have no clue what would make the brake drag more when you turn to the left... but here's a silly question: Is the front wheel loose? Rotor loose? Anything come to mind that might cause misalignment when you turn? Honestly, I have no clue.

As for the pits and scratches on the piston... in my opinion, and that's all it is, as long as oil isn't getting past the seal, the scratches and pits must not be too bad. I've often wondered how some of the huge holes don't let oil pass, and have finally decided that as long as there's no pit, hole, or scratch big enough to span the width of the seal, it can't get past the seal. That's the only logical (in my mind) answer for me.

Now, back to that dragging brake. Did you happen to go back to step 1 (loosen the bleed screw to see if pressure and drag go away)? If the drag goes away with that, I'm back to the master cylinder letting the fluid back in and relieve the pressure.

How about a couple of pictures to spur the imagination?

By the way... just because I've cleaned a caliper or master cylinder, doesn't mean I should forget about doing it AGAIN. Last year I swore up and down that I had the carbs on my CB900 clean after the second time. I was in denial... I was done after the SIXTH time! New insulators, new plug wires, new coils... all money wasted because I didn't get the carbs clean the first five times. Same can go for calipers and master cylinders, at least in MY garage. lol

 

[Stoney]

Well I managed to get this sorted out in the end!
First issue was that the master cylinder was shot, allowing air back into the system. I decided to replace the whole thing with a brand new reproduction one from Dave silver, this gave me the braking power I needed.
With the rubbing issue I ended up trying a lot of stuff, including forking out on a new piston. This was unnecessary. I eventually pulled out the little rubber ring inside the caliper and cleaned out the groove that it sits in (which was VERY gunked up). I then replaced the rubber ring with a new one and I am getting no rubbing at all.

Thanks a lot for all your help man, helped me slim it down! Now to fix some other shit...

 

[tommycarl]

Well I'm glad you got it sorted out. That gunk behind the caliper seal is pretty common... I'm sorry you had to go through all the extra work before finding out that was the issue. IN the end it apparently works well now, and you're now the go-to guy for the next fella with the problem. WooHoo!!