Topic: Still having brake trouble with cb750... help please

[Zim]

Alright, I'm getting pretty frustrated here with my front brake setup and fear that I might just need a new caliper all together.  The bike is a 72 cb750k with stock caliper.  I put a modern nissin 14mm bore M/C on the bike along with stainless brake line bypassing the pressure switch and getting rid of that thin steel line that is on the bike stock.  I'm almost certain I've bled the brakes correctly, as I have very firm pressure on the lever, however, I get terrible braking out of the front brake.  My rear drum brakes 20x better than the front, and I can't imagine that is anywhere close to normal. 

I don't know much about what the previous owners have done to the bike, so today I bought new pads and put those on there hoping to alleviate my problem.  While I had the caliper apart, I pushed the piston all the way out of caliper, cleaned the caliper and piston up, checked the seal (looked good... no cracks/rotting, leaking, etc) and put it back together.  I put some silicone based brake grease on the back side of the pads as well as the sides where they move inside the calipers.  Hooked everything back up, bled the brakes again to where I have a firm handle, yet STILL I have very poor braking from the front end.

Anyone have any other ideas?  Much appreciated.

[Zim]

The bore is exactly the same as the stock M/C, and the MC I'm using was brand new.  Can't imagine that's the problem.  I was advised to pull the seal out and clean the groove that it sits in from someone else... might try that and see what happens.

[steam-powered man]

also part of the braking equation is pad material; am curious if you used oem pads, organic pads, semi-metallic, sintered pads, etc?

[Zim]

The brake had the same feel with the stock MC that was on it when I rode it home.  That MC was leaking, so rather than rebuilding it I just put something more modern on.

The pads are brand new EBC  "FA" pads.

[ekpent]

Cleaning the seal would make the piston in the caliper retract better after braking but I do not think it will help much in the overall stopping power. Might be an issue if the front brake was sticking or dragging.Usually is the problem when the seal has crud behind it.

[dave500]

if the disc is very polished looking itll need to be deglazed.

[fasturd]

The Nissin 14mm master works very well with a dual disc conversion. I have used it many times on both CB750s and GL1000. I have also used the same 14mm master on a CB750 single disc stock set up and in that application it worked perfectly.

I by passed the pressure switch and also got rid of the hard line and went directly from master to caliper.

It is critical that the groove the seal sits in be PERFECTLY clean for best function of the caliper regardless of the master. Any crap in there at all can cause the piston to drag which causes all sorts of problems.

Make sure the rotor is clean and not glazed from the previous pad.

I have used stock old pads, new pads and "other" pads. I have had great luck with any of them as long as the caliper has been properly cleaned. The caliper swingarm has been cleaned and lubed and the master is functioning properly.

Now remember "GOOD" for a CB750 even with a new modern master is not the same as "GOOD" for a modern sport bike...
Though I have no problem 2 finger braking and getting my front wheel to howl on hard stops with the above single or dual disc set ups. Nothing beats modern technology and proper matching of masters and calipers.

[alfego]

I have a couple of old Triumphs with front disc brakes.  Vintage Brake in Sonoma, CA helped me with the same kind of problem Zim is having.  I bought two iron rotors, Ferodo Racing Brake pads, & 1 14mm stainless master cylinder, & stainless steel braided brake lines.  I left the stock Triumph Master Cylinder on one.  They are both able to stop WAY better than before.  It is attributed to the iron rotor.  The stainless or chrome rotors are no good for good pad adhesion or grip if you like, to the rotor.  The stainless rotor on my '74 CB750 is causing the same problem as Zim's, IMHO.  Rebuilt M/C, Caliper, new stock pads, no stopping power, I have stock old brake lines, and they could be swelling due to age, but the front brake is weak & not trust worthy, to say the least.  Iron rotors, make a huge difference, that's what all the vintage racer's use.  If I keep this bike, I will get an iron rotor, stainless brake lines, and Ferodo racing pads. 
If anyone is going to Barber's 7th Annual Vintage Motorcycle Festival, I'll have my 74 there for sale in the Swap Meet section.  Come on down and enjoy a great event.
Alfego

[lrutt]

My money is on contaminated pads. If they are not perfectly clean as well as the disk it won't stop for crap. try new pads and use brake cleaner on the disk.

[Zim]

The pads are brand new, the MC is brand new, the brake fluid is brand new (dot4), the SS brake lines are brand new.

I'll start with the cheap stuff and get a new seal in the caliper, make sure that groove is clean, etc.  How do I deglaze a rotor?  I don't know if it is glazed, but I may as well deglaze it...

[tango911]

what brand are your pads?

[ekpent]

You could wet sand it with some #320 black wet/dry sandpaper to deglaze and clean.

[Zim]

what brand are your pads?

They're EBC's

[Zim]

The brake line and fittings are all Kelly brand stuff, and the MC is a Nissin.  I did not buy cheap parts, but rather spent some time researching the size MC piston bore I needed, and replaced it with that.  Stock is 14mm, this one is 14mm.  As I said earlier, the stock MC had the same issue.  The stock one was in need of a rebuild, and I wanted modern parts, so I bought this one instead.  As mentioned above, others have used this exact MC on single brake applications and it works fine.

So... assuming MC and brake lines are good, where else would you guys start?  Im gonna go deglaze the rotor and see if that helps, but past that, considering the brake had these same symptoms with a stock MC and stock brake lines, I'm ruling those two variables out for now.

[phil71]

i just went back through the thread back to the beginning.  There's a brand new master cylinder, new steel lines, both of which are a big improvement over stock, or should be.
The problem is the same as with the OEM setup, and the caliper has never been rebuilt or changed?
  If I have this right, it can only be the caliper or the rotor, and unless that rotor is covered in oil, it's really got to be the caliper.

[Zim]

rubbed the rotor down with some steel wool and windex.  Seemed to help a little bit, but I still don't get get braking from the front end.  Handle is nice and firm, but the brakes seem soft.

[steam-powered man]

check M/C performance and/or plugged brake line:   can you loosen the bleed nipple and check fluid flow? 

stuck piston:   remove caliper and check for easy piston movement w/each lever squeeze 

brake mounting pivot frozen:  check 

flakey/pinched/internal fooey new brake line:   if you still have the stock llines, can you put 'em back on for a comparison test of your new brake line? 

new pad material too hard:   if you still have the stock pads, can you sand them clean and try them?

pad alignment:   full contact of the pad on the disc?   

[Zim]

check M/C performance and/or plugged brake line:   can you loosen the bleed nipple and check fluid flow? 

stuck piston:   remove caliper and check for easy piston movement w/each lever squeeze 

brake mounting pivot frozen:  check 

flakey/pinched/internal fooey new brake line:   if you still have the stock llines, can you put 'em back on for a comparison test of your new brake line? 

new pad material too hard:   if you still have the stock pads, can you sand them clean and try them?

pad alignment:   full contact of the pad on the disc?

When I loosen the bleed line, it flows straight fluid with no bubbles on every slow squeeze of the handle.  When I pulled the piston out of the caliper, I used the the lever to pump it out.  It had no sticking issues.  The pads are definitely making full contact, and the mount pivot was re-oiled when I pulled the caliper off initially.  I dont have stock pads or lines anymore...

One thing I am noticing is pushing it I get a high spot on the rotor.  Not enough to really drag the bike back, but I definitely feel it rub more.  I'm thinking it might be a warped rotor at this point...  Thoughts?

[ekpent]

rubbed the rotor down with some steel wool and windex.  Seemed to help a little bit, but I still don't get get braking from the front end.  Handle is nice and firm, but the brakes seem soft.

Not that it will probably make that much difference but steel wool would probably just shine the glaze and not remove it much. You need a more aggresive substrate like the #320 wet/dry sandpaper with water and maybe a dap of soap as suggested earlier to really help refinish the surface.

[steam-powered man]

i'm out, next?

[ekpent]

i'm out, next?

  Me too,First drink is on me at the Bar   

[Tanqueracer]

I deglaze my stock rotors with a 180 grit 3" flap disc on a 90 degree air die grinder, being sure to keep the disc flat. I run the grinder at a slow speed. This puts lots of small "scratches" in the rotor and makes for better "bite". Stock EBC pads. After 3-4 k miles the rotor starts to "shine" again.
I'd put the pads face down on some rough concrete and rub them around good until they are pretty rough. We do this at the race track between sessions and it helps loads. Breaking the glaze on both should make a big difference.  
Don't worry...the stainless rotor is so damn hard you ain't gonna hurt it no matter what you burnish it with (unless you do something dumb). It really is a poor choice for rotor material. Whoever said to use high carbon steel rotors is right on top of the problem.
Good luck

[Zim]

Put the stock components back on and you will have stock performance from a stock system.  Did I say stock?  I'm out, wait for me guys!!!

I've had the stock stuff on before the aftermarket, and it did the same thing... as I mentioned about 3-4 times in this thread.

[tango911]

Put the stock components back on and you will have stock performance from a stock system.  Did I say stock?  I'm out, wait for me guys!!!

I've had the stock stuff on before the aftermarket, and it did the same thing... as I mentioned about 3-4 times in this thread.

at least you have people trying to help you out.     cb750's are known to be on the weak side for front brakes.    Try drilling the rotor and cleaning it and rough your pads up maybe. 

good luck

[voxonda]

IMHO one will, almost, never get satisfactory brake power from the oem setup. I trackrace these bikes and it was "hell on wheels" when you were dicing with other riders with decent brakes. Tried almost everything, new mc, new pads, drilled or slotted disks. All to no availle. Until I threw out the garbage and mounted a 16 m/m mc from Grimeca and P08 Brembo calipers and racepads from Ferodo. Now it has real stopping power!

Good luck.