Topic: Front brake woes ad infinitum

[Saperlipopet]

Hello, I have a 1978 cb750k with the worst front brake issues I've ever run across. It's been about of month of research on this forum and on others trying to figure out why this brake is still awful. So far:

-Master cylinder rebuilt
-New OEM brake line installed
-New caliper piston (old one was pitted)
-New caliper dust seal
-New brake pads
-Bled brakes ad nauseum using mighty vac hand pump
-Adjusted the caliper arm so that pad B just BARELY rubs the disc when spinning wheel freely
-Tried the magic "tying off the lever overnight" trick (multiple times) still spongy as hell in the morning

Pretty sure there's no air in the line. I can see the action down at the caliper. When I squeeze the lever the piston moves to meet the rotor, but then the caliper sort of "twists" up and away from the disc when I squeeze harder (like panic braking) instead of bearing down on the rotor like I feel it should. It also manages to drags and squeals intermittently (usually under braking or after heaving braking).

I feel like such an idiot. I've thrown hundreds of dollars in new parts that have honestly done nothing to improve the stopping power. I can still squeeze the lever all the way to the bar and keep rolling and rolling and rolling...

Some people tell me it's just the terrible early disc brake design and I'd be inclined to believe them if I didn't have a friend who has a '74 750 Four with a SLIGHTLY different brake design (his bleed screw faces forward, mine faces outwards but they're both two-bolt single piston calipers) that brakes PERFECTLY compared to mine.

I saw someone on this forum do a swap for KLR650 front caliper on his cb550. I'll try anything at this point, I love everything else about this bike and if I could just make it stop properly it would be perfect!

[thep1pe]

Is the brake arm mounted directly to the fork then the fender goes on top then it is bolted together. If the fender goes on first the pads will engage at an angle causing poor brake performance and pads will wear at an angle.

[Saperlipopet]

Is the brake arm mounted directly to the fork then the fender goes on top then it is bolted together. If the fender goes on first the pads will engage at an angle causing poor brake performance and pads will wear at an angle.

This is something I've been meaning to check but without a visual reference I can't really understand what you mean, so here's some close-up shots I took of the brake setup. I'll take better ones tomorrow.

https://imgur.com/a/5YO5XXd

come to think of it, maybe the spring at the adjustor is worn and doesn't push hard enough against the piston?

[thep1pe]

You seem to have the brake assembled correctly. I use sintered brake pads for better performance. Try to ride the bike and bed in the pads.

[Saperlipopet]

And, have you confirmed the small return hole in the MC reservoir is clear? If the fluid can’t cycle, it won’t build pressure.

I took the master cylinder apart and used 110psi compressed air through it.

Here's a question, with the reservoir cap off, if you yank the brake lever back quickly should it shoot brake fluid up into the air? I've heard some people say this means there's still air in the system namely the small return hole mentioned above might not be clear, so maybe my next course of action is to empty the master cylinder of fluid and make extra sure that little hole is clear with a piece of guitar string or something...

[evanphi]

If your MC needed rebuilding, is it possible there was corrosion in there as well? The bore for the MC cyl has to be smooth as well. Pitting could be sending fluid around it, meaning spongy feel.

[scottly]

Here's a question, with the reservoir cap off, if you yank the brake lever back quickly should it shoot brake fluid up into the air?

Yes, that is normal. It means that tiny hole is open. Take a pic looking from the side. BTW, your fender brace is installed on the correct side of the caliper bracket.

[AwesomeCB]

You have washers on the bolts and bolts are tight? Happened to me once...

[Saperlipopet]

If your MC needed rebuilding, is it possible there was corrosion in there as well? The bore for the MC cyl has to be smooth as well. Pitting could be sending fluid around it, meaning spongy feel.

Definitely possible. The MC was in terrible shape after years of sitting.

Its far easier (next time) to simply buy a repo MC. It guarantees the orifices are clean and assembled correctly, and they run about $40 at DSS.

I could not agree more, 40$ is close to what I spent on the rebuild kit

You have washers on the bolts and bolts are tight? Happened to me once...

Yes

It's good to know the bracket is assembled correctly. I think my next course would be to try a different MC, especially one with a smaller bore.

[scottly]

Take a pic looking from the side.

[scottly]

It's good to know the bracket is assembled correctly. I think my next course would be to try a different MC, especially one with a smaller bore.

Just replace it with a stock report and go riding  No need to tinker with bore sizes since your MC is probably knackered.

What leads you to believe it's a problem with the MC???
There is something that doesn't look quite right to me in this pic; anyone else spot it?
 

[jaytee-nz]

The brake pads are on an angle meaning the caliper is not square to the rotor.

[scottly]

True, and also the piston side pad is worn down to the red line, and worn at an angle. There's something else.

[754]

Cant tell for sure but it may have a 500 rotor on it.
 I have made that mistake, and the brake really sucked.

[scottly]

That's what I was thinking; 500/550 rotor with a 750 caliper. The gap between the outer edge of the rotor and the underside of the caliper looks too big.

[Saperlipopet]

Thanks for the replies everyone. I'll take some better pictures today out in the sun from multiple angles after work.

[greenjeans]

As much of a pain in the arse as it is...  I'd double-check the orientation of the parts in the MC.   I've done many, many MC rebuilds over the years and I had to re-do the last one I did because I wasn't paying attention and had one of the rubber parts on backward.    Since you have new lines, seals etc.  you should get a rock hard lever.    I'd take one last look at how the master cylinder plunger etc is installed.

To clean the bore of my MCs, I take a drill bit and spin some 0000 steel wool onto it.   Kinda gives it a very light hone to the bore.  Then I clean the holy hell out of it.

Also - was the rebuild kit from Honda or aftermarket ?  Not terribly unusual to get a very similar kit that doesn't quite work as well.  I try and re-use the plunger that is original to the MC as well.

[754]

Yes, measure rotor diameter.
Itvreally affects braking power.

[ef9tuning]

 In the front view of the caliper, its looks like the lever is being squeezed, which would lead me to believe the brake lever would feel very firm... I suspect the issue still lies in the MC

[Saperlipopet]

Also - was the rebuild kit from Honda or aftermarket ?  Not terribly unusual to get a very similar kit that doesn't quite work as well.  I try and re-use the plunger that is original to the MC as well.

It was an aftermarket kit I believe, from vintagecb750.com (a supplier here in Canada). I'm quite confident I put it back together correctly since I tripled checked everything, but it would be worth it to take it apart again and look through it... Only once I've ruled out my rotor/caliper situation, though. Honestly, I should have re-used my plunger as well since I read about a slight (like 1-2mm) difference in length in another post somewhere which led to poor brake performance.

Here are some better photos of the caliper.
https://imgur.com/a/ZfK0dzU

I was able to shoot at a wider depth of field because this time I shot the pictures in daylight. Tried to get every possible angle of the thing, but if there's another angle you'd like to see let me know and I'll shoot some more!

Even when resting, to me it looks like the caliper is crooked, as if the pivot arm was bent or something. It stands to reason that if the caliper is compressing at an angle it has very little surface area and would not do much to slow the bike down.

[thep1pe]

The pad on the pivot arm seems as if it's ready to fall out. Have you checked if the retainer is located properly. Also it's worn at an angle. There is no way that would work properly.

[754]

Measure the rotor, it can't be that hard..

[scottly]

It doesn't look like a mismatched rotor/caliper in those pics, but I think I see the cause of the caliper misalignment; I'm guessing that rotor "fender" is mounted underneath the head of the  bracket pin, spacing it inward and cocking the rotor.
 

[Saperlipopet]

It doesn't look like a mismatched rotor/caliper in those pics, but I think I see the cause of the caliper misalignment; I'm guessing that rotor "fender" is mounted underneath the head of the  bracket pin, spacing it inward and cocking the rotor.

I think you might have it, can I just remove it?

[scottly]

Yes, remove it, and also remove the adjuster screw and spring, at least until your brake problems are corrected.