Topic: Can't get the rear brake to work, cb 750 - 72

[persson-johan@hotmail.com]

Hello!
Im in the middle of assembling my cb 750 - 72 ive been overhauling for two winters but im stuck with a weird problem with the rear brake.

The problem: When the rear wheel is mounted there's not enough adjustment to make the wheel brake. It barely brakes even with the nut fully tighten, and ive tried with different positions of the wheel - shortest wheelbase - longest wheelbase (shouldn't matter anyway). The brake pedal goes all the way down.

I never drove the bike or tested the brakes before tearing the bike apart so i don't know is it worked before. The brakes ar all cleaned and taken apart. The drum is well within spec. and so are the brakepads with about 4,5 mm left. The cam doesn't rotate to much that would indicate worn pads (see the pic) I have checked and triple checked everthing but i can't see the problem so im asking you guys for help!

Here are some pictures on the setup and i also took some measurements on the brake arm and brake stopper arm, maby i have parts from another year/model? Hope you guys can see something i can't

Brake arm (the one with adjuster nut) = 59 cm or about 23 1/8". End to pin hole centrum.
Brake stopper arm = 36.3 cm or about 14 1/8". c/c






Regards
Johan
Sweden

[Chachi]

Do you have factory stamps on your brake actuator arm (top pic) so you know where to set it on the teeth/spindle? I have them on all my arms... rear brake, shifter, rear brake pedal.

[Mitchell]

Sounds like its either the arm on the brake drum or your brake pedal is in the wrong position maybe.

[seanbarney41]

Yup, match up the dots stamped into the splined spindles and the mating arms and then there is  adjustment available one sline in either direction.  The dots can be hard to see.

[persson-johan@hotmail.com]

I have the dot and they are lined up!

[754]

 Wrong length torque arm, perhaps ?

[persson-johan@hotmail.com]

Wrong length torque arm, perhaps ?

Might be the problem! Looks stock and hasn't been cut tho, but who knows. Anyone have the c/c length on that? Thats the one i call "brake stopper arm" with the measurements in the first post.

[MoMo]

You can move the brake actuating arm from "normal" position rearwards one tooth that may give you the necessary leverage...Larry

[persson-johan@hotmail.com]

You can move the brake actuating arm from "normal" position rearwards one tooth that may give you the necessary leverage...Larry

I don't think that would be enough, i barely get any brake action when the pedal is all the way down. And it shouldn't be necessary with all the parts within spec.?

[MoMo]

Could you post a close up of the brake actuating arm and pivot?

[754]

 Brake stopper arm appears to be right length, could be the rod...forgot to measure that.
You could run a spacer to take up some of the slack.

[persson-johan@hotmail.com]

Thanks! Yes a spacer on the rod would probably solve it, but I'd like to find out what is wrong

I'll take some close up pictures on the pivot arm next time I'm in the garage.

Thanks for the help and answers!

[persson-johan@hotmail.com]

Ok! Here are some pictures:

All the way back:


Here it starts to brake:


Adjuster all the way in...:


Pedal still waaay to far down and barely brakes:


Brake arm:


Notches lined up:


Here also:


Hard to get a good photo of this but here's a try:


I can't get my head around this..! Only explanation is a brake push arm that is too long. Anyone got any measurements on that? But yet i don't think the PO would have ridden around like this..

Thanks for the help!

[Stev-o]

My guess is you have the wrong part on there. What is the overall length of the rod?

[evanphi]

That's all it really could be, the rod is too long. Everything else seems to be in place where it should be.

[setdog]

I just did mine the other day, I took the foot peg off and rotated the brake pedal arm back a few teeth, The marks on the brake pedal don't line up but I have nice feel and the fin on the actuator where the rod connects has no interference.  Then I put foot peg back on. 

Took me several attempts to get it feeling right.  Seems a few times it looked good, felt good but was ridiculously squishy, so I had to keep fiddling.

[enwri]

77 CB750k,
Brake rod, 550mm to the saddle,

Brake stay 325 mm, centre to centre.

I have a shorter brake arm/lever 20mm shorter

[2wheels]

Just currious -
Do you have the chain on and adjusted?
What happens if you push the wheel forward as far as it will go?

[MoMo]

I would pivot the actuator arm back one notch . I've had to do that on several bikes

[theslayedsaint]

I would pivot the actuator arm back one notch . I've had to do that on several bikes

+1 had to do the same thing with my bike.

[persson-johan@hotmail.com]

Thanks for the replys!

I'll try and move the pivot arm one notch.

Did you measure up to the part that connects to the brake pedal? I had 590 mm from the end to the centrum of the hole where the brake pedal connects. I'll measure only the rod if that's what you did!

No chain on the bike and I have tried pushing the wheel to the end points, no difference.

I don't want to rotate the pedal while the peg is of. If you do that the angle of the arm get steeper and the leverage will be less.

Really appreciate the answers!

[enwri]

Did you measure up to the part that connects to the brake pedal? I had 590 mm from the end to the centre of the hole where the brake pedal connects.

Just measured to the bottom of the saddle by sliding the tape up along the rod until it hit. It's dirty in there, swingarm grease overflow I've squished into the brake rod pivot saddle.
With new shoes, had to move it a notch in the other direction. Now they are worn in I put it back on the dots, drum or shoes worn, maybe a notch past.
Saw in the photos, the pedal stop adjuster bolt is threaded end down, if the end of the bolt gets mushroomed or bent it can be difficult to get out. My bolt is on the footpeg mount, so is different anyway. Don't know how yours should be, so, maybe just ignore that last line or two..

[persson-johan@hotmail.com]

I got it to work with moving it one notch back, sweet! I still have to adjust it quite much but there's much adjustment left.

Well now it works!

Also I primed the oil pump yesterday, got the pressure up to 60 psi on the starter, fitted some plugs, hit the starter and it runs!! Hasn't been run since 94 I think just waiting for a chain and then it's time to bed the rings in!

Thanks for the answers. Really helped me out this time!

[theslayedsaint]

Good news! Got to ask how are you measuring the oil pressure, did you buy a gauge that goes go the oil galley?

[persson-johan@hotmail.com]

I have a oil pressure gauge for cars and bikes that connects to the oil pressure light switch hole. I think I will have connected for the first run also. Later I won't have a gauge on the bike, only the stock oil light.

[MoMo]

Good news,  sweet feeling..Larry